I think that once you get your stash started and learn what you want/need for your projects, you will notice more opportunities and be in a position to take advantage of them. I shop garage and estate sales, as well as trade fabric with my guild friends. Today I traded 2 rolls of furry fleece that I got for $5/roll at a custom costume store close-out with a friend for a pile of wonderful quilting fabrics. She had also been to the same store sale and purchased furry fleece. Now 6 months later she has found someone wanting to buy a bunch of the stuff, so asked me if I had any to sell to her. I suggested a trade for quilting fabric. It was a win/win situation, and I left with at least $100 of beautiful fabric. She said she felt like she was the big winner.
Building a stash is so much more fun when you can do it with the help of friends!
Dayle