I have used the cheap (like 2-3 dollars a yard), plain fleece at Wal-Mart or Joann's. If it's not wide enough I will zig zag stitch it together. It's very easy to sew through though, so I might not use that for both the quilts you are making if you are looking to practice with them. You might want to use it for the first just to get used to the motion of quilting, but then you might want to bite the bullet and get batting (maybe using a coupon at joanns) that you'll normally use for quilts to do the other. I have found that if you don't use the real thing for practice, it's not the same thing once you're ready to really try it. Like I have tried practicing on a 12 inch square, and I do great! But that's a whole heck of a lot easier to maneuver than a full size quilt, so while it's good practice for the motion, it's not good practice for the actual feel.
Good luck!
Rachel