My budget might not be as tight as some but I only buy full price for a very, very special quilt.
I have my list ready for my annual trip to the Lancaster PA Show(next week, yeah!). Don't usually buy fabric from the show venders but we do stop at the many fabric shops in the area. They have top of the line fabircs at about $6 per yard but first I head for the Sale Section. Last year at Zook's I picked up over 7 yds of a great fabric at only 79 cents a yd. OK, that was a one time thing because most of the yardage I bought was at 2.99. Picked up 5 yds at that price of a great beige print for backgrounds. Saw the same fabric at my LQS for 10.98. Joann's remants are 50% off the price on the label and I usually pick up some whenever I'm there, just make sure the cashier gives you that 50% off. Haven't success at Thrift shops but that doesn't stop me from checking them out.
My Guild makes tons of pillowcases for kids in hospitals and there is usually 2" strips leftover that I use for scrappy log cabin and string quilts. Guild also has a big stash that members are welcome to add to or take whatever they want or need. Most of it is old and sometimes thin but I like the thin fabrics as foundations for my string quilts. Of course, I give the fabrics a really good washing and dry them in the dryer.
Someone suggested posting that you want cotton fabrics on your local FreeCydle message board. May in Jersey