I have bought fabric by the yard from Amazon and Fabric.com I have also found some incredible prices on flannel sheet sets on Amazon. (finding the right color at the right price is the catch there). Recently I had a 30% at Kohls and I found some king size flannel blankets for $23. 90" x 108" is perfect and since they are all season blankets they are basically sheets. I am very happy with them
I just keep my eyes out for sales and good deals - - before I need it for a backing
I do like the idea of buying a bolt just so I wouldn't have to worry about it... I might shop around and see what I can find just for fun