I'm impressed with the quality of the flannel sheets from LLBean. They are available without having to get the whole set, and they come in quite a selection of colors.
I would want to start with a new sheet. After spending all the time and money on the quilt top, I would want a backing that is fairly new, and still has a lot of life left in it. Not to say that you couldn't find that at yard sales, but where I'm from, flannel sheets are pretty worn when they hit the yard sale market.
M