I've taken to buying used top sheets at Good Will to use as backings. (Many of the top sheets are in super condition because a great many people don't like to use top sheets.) they work very well with my HQ16/PCquilter set up. If I find top sheet that is in good shape and a 'perfect' backing for one of my non-charity quilts, i will buy it and use it.
I also have a stash of flannel sheet sets that I purchased at Sears a couple of years ago. They were very 'price reduced', and the quality is good. Choices were limited as it was a winter end of season sale - and we all know that no one stocks flannel sheets in summer colors -ever.
The flat sheets are, of course the easiest to use, but the fitted sheets can be 'unfitted' and cut to use on a decent sized lap quilt. (scraps will eventually wind up in a scrap quilt.) The pillow cases are a bonus as the pillow case can be given to the recipient with the quilt.
It's also nice to use fabrics from my stash to piece a backing, which has the affect of being a two sided quilt. I especially like to do that on baby quilts because I have learned that sometimes children of 3 or 4 don't like to use a "baby quilt", and, at the same time, don't want to give it up. A 'grown up' side means that they can show the grown up side to their friends and cuddle up to the baby side when they need to. That's my theory, and I'm sticking to it!