Not always but sometimes I will use bed sheets. Twin sheets for the size quilts you are making should do it and with less cost. I also watch for sales at JAF with the bleached Muslin. I will buy a bolt at a time and it last a very long time. I also watch fabric.com because they too offer some good sales.
The other place I look is in my stash where I can sew a panel on the back and then piece together the backing using 45" wide fabrics. My stash gets a lot of charity quilt work.