I'll second all of these sites. Love Connecting Threads and Marshall Dry Goods, Marshall Dry goods is "cheapest" but all the fabrics I've bought there weren't, if you know what I mean.
I like Thousands of Bolts, but REALLY wish I could see more than 36 fabrics on a page. It takes so long to go through the pages, that I just give up.
Fabric.com is pretty good, too.