Ebay - Probably the best selection and best prices for hard to find fabric like knit prints. Here are some specific eBay Stores that I've found have good selections:

• Cute and Unique from cankorkmaz - Cute novelty print knits, and in my experience, you get a very generous yard (my cuts are often more like 50".)
• Purple Seamstress - Another seller with a good eye for novelty knit prints - plaids, retro, and more. Last time I ordered I got some sweet free stuff, too.
• Lycra Prints - Just like their name says, lycra prints, and lots of them for really reasonable prices. Their inventory isn't well labelled, so you kind of have to go through everything to find what you want. Many of their fabrics are chlorine resistant, making them ideal for swimwear.
• Ellet Closeout Fabric - Good selection of printed and solid knits.
• uBet uWin - A few prints, but this is a good store for getting bulk amounts of solid jersey knits for cheap.
• Eileen's Fabrics - Knit and woven novelty fabrics.
• Nae Girl - Stripes and novelties.
• Fabric Junction
• Fabric Addiction
• Kazoots Kidwear Outlet
• Debra's Doll Dreams
• Show Me the Fabric
• NR Fabric
• Fabric All
• The Fabric Collection
  

A little bit of everything:
Fabric.com - Discount fabric, including knits and faux fur. Tip: Fabric.com's selection constantly changes, so if you see something you like, grab it before it's gone.
Denver Fabrics - Large selection of fabric (including a lot of lycra prints), patterns, and notions.
JoAnn.com - Fabric, craft supplies and more. In case you haven't checked in a while, they've hugely increased the fabric available online... decent clearance section!
Sew Shoppe - Knit prints, notions, diaper making supplies and patterns - check out the Pretty Pads pattern!
Hancock Fabrics - Fabric and sewing supplies.
Wazoodle - Knit and woven fabrics and sewing notions.
Sewzannes Fabrics - Unique and funky fabric prints.
Lucysfabrics.com - Lots of printed knits and trims.
Fabric Master - Very cheap knit solids, other fabrics available as well.
Fabric Fairy - Really cute knit prints and more.
JCaroline Creative - Fabric and other supplies.
Spandex House - Lots and lots of spandex prints. You have to phone in your cc info, and I think there's a minimum order.
Mainstreet By Mail - Specializes in knits. Catalog is a bit tedious to navigate, but they have some good stuff.
Kids Fabrics - Mainly intended for kids clothing, small selection of knit prints.
Lyle's Fabrics - Fleece, knit and woven fabrics.
Zoozads - Some cute woven and knit fabrics, plus diaper making supplies.
SewBaby - Stretch knit prints and patterns for babies and kids.
Theatre House - Their fabric section has lycra prints, stretch fishnet, and black light / UV reactive fabric.
Nick of Time Textiles - Wholesaler with good knit selection.
Gorgeous Fabrics - Good selection - knits, woven, silk, suiting, etc.
CraftConn - Fabric and notions, awesome sale section.
Trendy Fabrics
Sy Fabrics
EZ Knit
Spandex World
Crybaby's Boutique
DollarFabric
Fabric Lane - Knits, corduroy, poplins, patterns, Ottobre, and size tags.
Bunte Fabrics - selection of European imports, many of which are organic. We also offer a large inventory of European patterns, ribbons and buttons.

Mostly 100% cotton, quilting fabrics:
eQuilter - Ridiculously huge amount of quilting fabric.
Repro Depot - Really cute retro and vintage reproduction fabrics.
Fabrics-Store.com - Huge selection of linen and cotton.
5 Bucks a Yard - That's right! Your favorite novelty and quilting prints, all $5 a yard (or $3.99 and less for the sale section)!
J and O Fabrics - Good variety of novelty prints.
Hancock's of Paducah - Quilting and home dec fabric.
Fabric Direct - Huge selection of coordinated polka dot and stripe woven fabric and knit solids, available by the yard or by the bolt.
The Virginia Quilter - Quilting fabric and supplies, lots of dots and stripes.
Quilt Knit - Lots of quilting fabric and supplies, with a pretty extensive sale section.
Buy Fabrics Online - Dedicated to carrying Alexander Henry prints.
Kitty-Craft - Specialty fabric shop with really cute novelty japanese prints and craft books.
The Tattered Rabbit - Nice quilting fabric selection.
Fabric Attic - Quilting prints and patterns.
Mendel's - Cotton prints, oilcloth, faux furs, and more.
Cheap Threads - Novelties and other good stuff - patches, buttons, etc.
Tonic Living - Cute selection of retro fabric.
Crafty Planet
Sew Natural
The Cotton Shop
Super Buzzy
Triple E Crafter Fabric
FabricParadise
jelowe.etsy.com
Fat Quarter World

Other interesting fabric stores:
PM Organics - Organic fabric, with some really cute printed knits.
Plush Furs - A whole ton of faux fur fabrics.
Melinamade - Expensive but awesome retro inspired fabrics.
Minky Delight - Lots of minky fabrics and some cute vintage inspirited prints
Fabric Hound - Unique European imported fabrics.
Fleece-Fabric - Bulk supplier (50+ yards) of fleece fabric.
MJTrends - Good selection of stretch PVC.
JacQue Textile Designs - Artist-made fabrics.
Wingham Wool Works - handspun and felt fabrics.

Ebay - Also a good place to find stuff like stretch lace, elastic trim, and novelty tapes - grommet tape, eyelet tape, d-ring tape, etc.

• Lucy's Fabrics - Grommet/eyelet tape trim available in small and bulk prices.
• Kathy's Lace and Elastic Outlet - Nice selection of stretch lace and fancy elastic trim.
• Look 4 Franny
• Gothology - Trims and laces, located in Australia for all of you Aussie DiYers.
• DIY-Lace-and-Trims - Lots of lace and trims
• Fabric Fanatic - Lace, elastic, and lots of assorted trims.
• FableWear - Gothic trims and embellishments.
• Indiscretion - trims!
• Mix and Match - lots of cute buttons
  
• Poppys Wicked Garden Too
• Adornia
• Lindy Laces - UK supplier
• Trims4Less
• Creative Effex
• Berry Patch

Another good source of trims is Etsy:

• trimgoddess
• Shrinkle - A variety of corset trims, including some rad colors other than the standard black and white.
• MTCoffinz - Lots cute assorted trims.
• theVintageDoctor
• Indiscretion
• MadScienceCreations
• fabricat
• Countessa

Lace Heaven - Wholesale prices, with no minimum order. Really good selection of both stretch and non-stretch lace, as well as other embellishments.
Shoppers Rule - Lots of sewing supplies, with replacement blades for sergers! They have a "make an offer" feature so you can haggle on prices!
Zipper Stop - Zippers with bulk prices available, plus other sewing supplies.
CTSUSA - Wholesale sewing supplies like elastic, thread, needles, etc.
JoAnn Fabric - Zippers, thread, fabric markers, etc.
Bead Room - Vintage glass beads, czech glass beads, lampwork beads
Lace For Lace - Wholesale supplier for lace, organza, and more.
CheepTrims - Wholesale supplier of trims, with a good selection of the harder to find stuff. They have a minimum order ($75, I think), and their stuff comes mostly in bulk.
Snap Source - Really great snap attachment tool. I reviewed it here.
Nancy's Notions - Lots of notions and sewing/craft supplies.
Ghee's Handbag Patterns - Bag patterns, hardware, and frames.
Studs and Spikes - Pyramid studs and spikes, and other hardcore embellishments.
Crust Punks - More studs and stuff. They have a fabric section, but please note that prices are PER FOOT (i.e. not cheap).
M&J Trimming - Lots and lots of trims and other embellishments, notions, etc.
Tebo Designs & Pattern Co. - Well illustrated patterns for decorative quilts, dolls, table toppers and more.
ZipZapKap - vintage sewing patterns
Cream City Ribbon - Cream City Ribbon is the only producer of 100 percent cotton ribbon in the U.S.  They offer beautiful, eco friendly ribbons.
lusciouslabels - Custom woven and printed labels, and size tags.
Craft Queen - Specializes in ribbons with more than 500 available. Also a wide selection of buttons, flowers and general scrapbooking supplies.

Dick Blick - Screenprinting materials, canvases, and more.
Mister Art - Discount art supplies.
Create For Less - Scrapbooking, needlework, and more.
The Swans Nest - Jewelry making supplies.
ComputerScrapbook - Digital and printable computer scrapbooking
Krafty Kids - Craft kits for kids, with a craft-of-the-month club.
Graveyard Mall - Closeouts on assorted craft supplies.
ScrapbookPal - Scrapbooking supplies with free shipping on orders over $25.
A Cherry On Top - Scrapbook Shoppe with organization by company, theme, and artist.
Scrapbook - Scrapbooking store with forums, gallery, and a library of free resources.
SilverCrow Creations - Lots of cool supplies for scrapbooking, altered books, collage, and jewelry
Mosaic Basics - Glass and tile mosaic supply.
SerendipBeady's Speciality Beading Tools - Check out the bead spinner!
Boston Clay Works - Polymer clay and glass bead making kits and supplies.
Lyle's Fabrics - Yarn and other misc. craft supplies.
Anita's Arts- Needlework, painting, rubber stamps, paper punches, and other craft supplies.
Photo Jewelry Making
SKS Bottle - Bottles and packaging, great for bath & beauty sellers
ULine - Shipping, packaging, labels.
Warehouse Craft Supplies - wholesaler of craft and floral supplies.
Quirks of Art - Scrapbooking and rubberstamps.
Crafters Pick Craft Glues - Carrying a full line of General, Fabric and Specialty glues. 25% - 40% off MSRP with free shipping in the continental USA.
Shop My Web - Undecorated tins and glass bottles.

Merch:
Hottopic - Check out the clearance section.
FashionPunk - Huge $9.99 shirt section
Every Thing But The Music - Band tees with free UK delivery.
Angry Young and Poor - Cheap band tees.
Interpunk - Ditto on the cheap band tees.
Machete - More cheap band tees.
Stylin Online - Variety of t-shirts, check out the awesome "costume" shirts.
MixUnit - Hip-hop t-shirts, most of them $20 or less.
Hollywood Mega Store - Movie, TV, and music t-shirts.
Spreadshirt - Create your own merch, or buy from other small designers.
Sourpuss - Band merch plus original designs.
Angry Young and Poor - Punk rock merch.
Machete Manufacturing - Nice clearance section, plus they've got a 3 tees for $15 promotion.
Tons of T-shirts - Licensed tees, novelty tees, etc.
JSR Direct - $16 band tees and a "$9.95" sale section
Punk Stuff - Cheap tees.

on Ebay:

• Wynn Co - 99 cent t-shirts!
• The Vintage Guy - tons of vintage tees.
  

Blank T-shirts:
Alternative Apparel
No Sweat
American Apparel

Unique/Art T-shirts:
Spraygraphic Apparel
Threadless
My Two Cents
Uncle Kimono - clothing line by John Malkovich?!
VanRockn Apparel
Dickey Shirts
Reggae Urban Fashion

Novelty T-shirts:
Nerdy Shirts - Good sale section with shirts for under $10.
A Shirt For A Stud
Thread Pit
Useless T-Shirts
This Shirt Rocks
Lucky Threadz
Brown Turtle
BeWild
Tshirt Humor
Crack Smokin' Shirts
Wicked Cool Stuff
A&E Designs
BustedTees
Look At Me Shirts
Shocker T-Shirts
Prank Place