I have quilts my Mother made... using old blankets; mattress protectors as batting and sheets as backing. They're still good!

Since I've started quilting, I've saved worn out flannel sheets and blankets that have "bit the dust". :o

I have used mattress pads, as well as fleece.

I think regular fleece can be a bit lofty, but those cheap fleece blankets that are sometimes given as promotions for a charity donation are thinner. I think batting is a good way to re-purpose those.