I've had bad experience with Fabric.com, but I'm a fan of eQuilter (easy to search) and Fabric Depot and FatQuarterShop.com.
I recently stumbled across The Patchwork Pig which is a shop in the Four Corners area. Turns out they were out of a fabric I ordered but they had one by the same designer/company that was nearly identical...they called me and offered to send it for the same price and take it back if it didn't work as a substitute for what I ordered! Now that's customer service. The fabric was a perfect substitute and I'll order from them again.
http://www.patchworkpig.biz/