Flannel sheets go on sale at stores shortly after Christmas. By March the prices are steeply discounted. You can buy King size flannel sets at extremely good prices, which also gives you matching pillow cases. If possible, get white or tan solid color flannel sheets so that you can use them for both batting or backing. I've gotten a few in plaids, too, and they look great as backings on the 'right' quilt.
A word of caution when using flannel sheets - because of shrinkage. they MUST be laundered at least once, preferably twice, before you use them.
Many people will tell you not to shop in Walmart, but I use their fabrics frequently. (And I have seen some Walmart fabrics in the quilts made by some of those 'many people'!) I made quilts for my grandsons 8 and 6 years ago. Both are in use and both are holding up fairly well. The binding is fraying, but that is as much from my lack of expertise and technique as from wear and tear.