I buy most of my fabric from thrift stores. In our area, you can get bags crammed full of fat quarters for 2-10 dollars.
I also buy sheets, pillowcases and funky shirts. Sheets can be used as background or backing or binding. I even gut comforters so I don't have to buy batting but then sometimes that limits my quilting options due to thickness.
Estate sales, garage sales, craigslist, freecycle and the closet.
I only buy new when I need something specific that I don't have on hand for a project.
If I didn't do this, I wouldn't be able to afford it. Fabric is expensive now!