Cotton sheets at the thrift store. Very cheap and you can find some beautiful sheets.

I have ordered Cotton Sateen from here, and she has great service! http://www.widefabrics.com/

Ditto, marshalls is a great place to start.

Fabric.com too. I really like some of their wide Batiks.

hancocks-paducah.com

they have sales on the wide backings, too. Their prices are pretty good. Their service is also good.

and, stitch-n-frame.net is another source I like.

Try Fabric.com - they have a large selection.