Lately I buy the 108" backing fabric because our little fabric store here in town has them for $6-$8 per yard. They don't get a large assortment to choose from, but they are good quality and when they have something I like I will buy two 3-yd cuts and stash it away for future use. Other than that I buy coordinating fabric and piece my backs. If I think things through before I make the quilt top I will try to find a fabric for backing that I can also incorporate into a few blocks or borders of the quilt. I don't mind mixing things up a bit!