I have used sheets for years, cottons if I can find them with a low thread count, cotton/polyester blend if they have a good "hand" feel, flannel flats if they are on sale and decent quality fabric. I do very carefully check the used sheets. If they have been used a lot they may not wear with the quilt top. If they are pretty crisp, not faded, and seem like they were more abandoned than used I will buy and use them.