I, too, use bedsheets for the backs of larger quilts. I buy them from Target and have never paid more than $17 for a king flat. If the sets are on sale, I'll buy them as well for two or more quilts, one with matching pillowcases. 200 - 250 all cotton is best, IMO. (I miscut one sheet for a queen-sized quilt and ended up piecing it with leftovers from the front. Turned out great.)