Places I have used:
Rochford Supply: The cheapest no-brand Cordura I can find. Also great prices on hardware and webbing, especially in larger qualities.

Outdoor Wilderness Fabrics Inc.: Great selection and good prices. Cons: no online ordering.

Places I have not used, but have looked at:
Rockywoods Fabrics: Prices seem a little high, but you can order online.

Seattle Fabrics: Prices seem high, but they have a good selection and some useful tips.

Quest Outfitters: Haven't really looked at this site.