Look for sales and try to stock up then. Fabric.com is a good place especially if you go to their quilting fabric, clearance section. Plus they will not charge shipping if you order at least $35.00 worth. I have bought material from them.
Garage sales are good too. Look for cotton material. I just got back from the Thrift store with over 15 yds of cotton material, ( brand new ). I have it in the washer at the moment.