Premium muslin is good. I have bought several brands of wide quilt backs on sale at fabric.com and thousandsofbolts.com and been happy with them. Right now I am working on several charity quilts and am using some nice used cotton flat sheets I bought at a resale thrift store for $3 each!! I also often piece backs using leftover fabric or coordinating color fabric from my stash and like that it makes the quilts 'Reversible'!