If you really want to try sheets for backing, go to a thrift shop and check out the sheets there, usually the fiber content is on the label. Try to get all cotton, also look for softness, you can ususally get king size sheets for under $5.00. Just remember that sheets are usually harder to quilt through because of the thread count and most quilting advice I have read says dont use them. If you want to bag and tie a quilt the sheets work fine.:D