I frequently increase my stash from goodwill and other thrift shops. Sometimes the fabric is hard to find. I have found it in bins in with the sheets and bedding, or folded over a hanger with the towels and curtains. I have increased my stash 3-fold over the last month by finding fabrics that way. Most are leftovers from some sewing project, but there are some good yardage pieces to be had also.
Another place to look is in the pillowcases. You can find some that are almost like new for 35 -50 cents each. These are usually good sturdy cotton, but just check the labels and strength of the material to make sure. You can cut a lot of pieces out of a pillowcase.