Ebay probably has the best selection and best prices for hard to find fabric like knit prints. Here are some specific eBay Stores that are some of my personal favorites:
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. I always get some sweet free stuff when I order, too.
Lycra Prints – Just like their name says, lycra prints, and lots of them for really reasonable prices. Their inventory isn’t well labeled, 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.
Fabric Addiction – gothic, punk, etc. novelty knit fabrics
Debra’s Doll Dreams – lots of novelty knit fabrics and more
Fabric All – knits, stretch lace fabric, etc.
Fabric By The Roll – By the roll and by the yard, nice selection of knit solids.
High End Fabrics – Lots of printed and solid organic knits!
Churn Dash Antiques – lots of vintage fabrics and textiles!
0ptima Fabrics and Collectibles – tons of lycra fabrics
In Fashion Fabrics – knit supplier for the UK
Outside of Ebay…
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!
Hancock Fabrics – Fabric and sewing supplies.
GirlCharlee – Excellent selection of knit fabrics.
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.
The 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.
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.
Fabric Lane – Knits, corduroy, poplins, patterns, Ottobre, and size tags.
Mostly 100% cotton, quilting fabrics:
eQuilter – Ridiculously huge amount of quilting fabric.
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.
Fabric Attic – Quilting prints and patterns.
Tonic Living – Cute selection of retro fabric.
Other interesting fabric stores:
PM Organics – Organic fabric, with some really cute printed knits.
Plush Furs – A whole ton of faux fur fabrics.
Fabric Hound – Unique European imported fabrics.
Fleece-Fabric – Bulk supplier (50+ yards) of fleece fabric.
MJTrends – Good selection of stretch PVC.
Wingham Wool Works – handspun and felt fabrics.