I have bought lots of clean but well used sheets at resale/thrift shops for quilt backings, and love them. They are very soft and are easy to sew through. Not to mention spending $3 for a queen size sheet makes that backing VERY reasonable in price. I usually only purchase solid colored sheets like white or ivory, but have had good luck with navy and black ones too when I can find them. I prefer a flat sheet, but have 'deconstructed' fitted sheets too.