A lot of times you can use left over material you my already have since they don't take a whole lot to make. If you are buying material for the first time, I would get at least a yard. It's folded over and it gives you extra to either make more or make mistakes.
Hopefully it's to make more. I have made sleeping bags/ cozies for my guys using flannel and cotton batting. I have a listing in the classified section of this web site for cozies and sleeping bags I have for sale.