All those are great but here are some of my other favorites:
5bucksayard.com or Hancock's have the best prices right now. Don't ignore your local thrift stores... they sometimes have great fabric. Estate sales, too. shopgoodwill.com also has occasional fabric prices and variety that make me seriously consider bidding for lots. The online store that tends to get most of my money is:
http://www.overrainbow.com/cgi-bin/Store/store.cgi If you have something particular in mind to purchase, try quiltshops.com to get a variety of stores and their prices.
The stash process is always fun!