I had really great results from Fabric.com. I found soccer players - so cute, and a Princess fabric for the back of 2 quilts. We have "Beverly's Fabrics" in San Diego and when I need a plain backing flannel, I use their flannel - it is marbled, so not a solid per se but they have such a variety of colors I usually find one that works. They have an on-line store.
Once I bought just a solid blue piece of flannel and it was the worst fabric ever - it bled like a son of a gun and had no body at all. It was from Joann's - usually the flannel I buy is good, you can tell by the feel, but that one piece was bad. I should have taken it back, but never did.
Right now there may be a lot of flannel sheets on sale at Bed, Bath & Beyond, Target and Walmart - maybe you could just cut them down.