Do you have access to a computer? Depending on where you live, it can help to check Craigslist and Freecycle online. Freecycle usually allows you to post one request per week for free items; you could request cotton fabrics for quilting.
Do be careful when making arrangements to pick up things. Never go alone.
Garage sales can be a thrifty source for fabrics too. Thrift shops where I am seem to overprice fabrics a lot, but some thrift shops are very reasonable. Sometimes at our thrift shop I will see some interesting bedsheets for very reasonable prices. They can make great quilt backs, or even be cut up for a pieced top. Our thrift shops have percent-off days every week too, plus "senior" days, etc. which makes the prices even better.
I would also make it clear to everyone that I really appreciate cotton fabric yardage for gifts.