Sounds crazy but you would be surprised what you can find at garage sale and don't forget your local goodwill or thrift store. We also have a Hobby Lobby in our area that has some nice fabrics.

Sandy

I don't know about other sewing forums, but here are some on-line fabric stores:
http://www.fashionfabricsclub.com/
http://www.discountfabricsusacorp.com/
http://www.housefabric.com/
http://www.designdivafabrics.com/
http://www.thefabricfinder.com/
http://www.homeportfolio.com/listing/fabrics.html
http://fabricguru.com/cart/index.php
http://www.onlinefabricstore.net/
http://www.moodfabrics.com

I used to get some good fabric deals on eBay, but it seems lately most of the fabric is from larger vendors that are quite expensive.
I check the local thrift stores, too.
Hancock Fabrics is pretty good and they have an online store. Check to see if you have a Mill Ends Textiles in your area. I like this store a lot, but the nearest to me is 1 1/2 hours away(sigh).

I'm with Sandy on the thrift stores and the yard sales. I buy flat sheets for $1-$2 each and use them for the backs of quilts. I buy jeans at yard sales and use the denim to make handbags. Full skirts can have a couple of yards of fabric in them. Suede skirts and pants can be used for appliques on clothing. I use old stained sweatshirts and pants to make covers for my floor mops instead of buying the disposable wipes.