I use flannel often that I buy at Joannes when they have their fall sales, and large selection, I cannot afford to use LQS flannel for backing. I realize the quality of a LQS flannel is better than Joannes, but unfortunately it is out of my price range for backing. I have used minky and fleece too. I also have bought flannel sheet sets that are good quality at my local thrift shop and yard sales and have used them as backing.
I typically use quilting fabric for backing when I want cotton.
I have read about using a flat sheet for backing too, which is made for several washes, would be durable, and less expensive than quilting fabric.