I love Moda flannel. Nice thick scrumptious flannel. Used on the back of a quilt for my niece. I used a marble but I've fondled others of Moda's and they all feel wonderful. Would highly recommend Moda flannel.

connecting thread flannel is lovely, not much shrinkage and doesn't pill, very soft. Have used as backing on a few quilts recently

Would a good quality flannel sheet work?

Most of the flannel sold at Jo Ann's is poor quality and it will pill after just a few washings. I use quilt shop quality flannel for quilt backs for that reason. I've bought wide flannel at thousands of bolts dot com, no problems with the quality there.
Sharon in Texas

What about Minky? for your back! it does stretch so you need to consider that when sewing but it's really nice fabric.

That's a great idea to use an LLBean sheet!

Thanks for all the suggestions. I haven't bought anything yet, but quilt doesn't need to be done until May :)

I love a flannel backing. I finished a quilt a few days ago and used flannel on the back. i washed and dried it first, but should have washing it multiple times since it bled (red). I was able to get most of the "pink" out of it by more washing and a final wash of Synthrapol , which I should have used the first time, and will remember from now on. I'm not sure about fleece shrinking, but what the hey, why take a chance!

I ordered flannel from Connecting Threads, since it was 50% off. If I don't like it when I receive it, I'll buy something else. Thanks for your suggestions.
Marcia