I, too, shop the thrift stores for fabric. They can be a goldmine, but it does take patience. My favorite is Salvation Army, their fabrics are always neatly folded and visable in plastic bags, and I think they are decidely cheaper than other thrift stores.
Garage and estate sales are also great, especially estate sales.
What the heck, ANY sale is good.
Don't be afraid to dig in the bins and search!