i don't know the best buy on backings RIGHT NOW, but they all have sales. watch for them.
now, one yard of fabric has 56 5" squares. if you pay postage for a few at a time, it might be better to get fats on sale for 99*. those go on online sale, at that price. the other thing you can do is start or join a swap here for the size you're looking for. say 6", 2 squares at a time.
pm with your address. i'll get you going, not for a swap, but for a starter to work with.