I can sometimes find yardage in thrift stores, but I also look at bed sheets, pillowcases, curtains, etc... Many curtains and specially valances are made with cotton. You just have to spend a bit of time "unsewing" them. (and valances are misleading because they don't look like they have that much fabric, but it's all doubled. Once you take out the stitching, it's surprising how much fabric is there).
Another place I look are linen stores. They sometimes have discontinued bed sheet sets (100% cotton). I've scored king sized sheet sets for $40. There's a lot of yardage in there! (considering my LQS sells cotton sheeting for $10/meter on sale). I just steer away from really high thread counts. These are really nice when you're looking for say, plain white fabric.