Hi- I have used flannel sheets for small, usually children's quilts. I also use them to put on the car seats so my dogs don't ruin the seats! I have gotten them at Goodwill, the Salvation Army and Amvets. You have to be careful and look them over carefully to make sure there are no tears or worn spots. I have had great luck finding them and have bought many. Of course they must be washed before use but they have lasted as quilt backings for a long time and cost a pittance.
Madeline in Buffalo :-)