Keep an eye out for garage sales and estate sales, look on craig's list, etc. Most of my stash has come from one woman who was DE-stashing! I got tons of fabric, books, magazines and batting for cheap cheap cheap! You can check out second hand stores like Salvation Army, or consignment stores. They sometimes have fabric, but you can also go to clothing! A bunch of xx-lg cozy flannel pj's will make a great rag quilt, men's cotton dress shirts, too. The bigger the better (more fabric!). I use flat bed sheets for backing and just pick up a couple when they're on for a good deal - MUCH cheaper AND it's plenty big enough so there's no piecing.
Also look for sales at your local craft/fabric stores, I never buy anything at my fabric store unless it's on sale (unless it's *very* important...which most stuff isn't lol). I also try to buy 1/2-1 yard (or more) extra every time I *do* buy for a project.