I have found
www.sellfabric.com to be an excellent source. I have bought from them on ebay although I think they may sell through their regular site as well.
Be careful, though--they often have cut lengths cheap that sound great. But they are often far inferior to their standard "by the yard" fabrics. When buying by the yard I have found their stuff to be consistently good, and after a few yards the combined shipping makes it reasonably-priced. And it's worth it to me to know I can count on their quality.
Their poly-cotton twill blend is good stuff to work with, and puckers less after washing than the 100% cotton.
Moosedog