See if there is a "recycle" or "scrap exchange" type store in your area. They take donations from both individuals and companies to keep things out of the landfill. There's two in my area. Recently I bought a wonderfully soft piece of flannel, eight feet long. It looked like it had just come off the bolt at a fabric store. It was only a few dollars. Once I bought twenty-four quilt blocks from the same store. I sewed them together to make two lap quilts. The blocks were 75 cents each and I was given a little more off because I bought them all.
I've noticed that some thrift stores will carry fabric (I mean like the flannel above, not just sheets), while others never do.
If you are in an area with a Craigslist, search there. You can do a search under "Arts and Crafts" but sometimes fabric will be under "Garage Sales" too.
Tate