I've used cotton thermal blankets (that had a hole in it) for the batting. That quilt had the most wonderful drape of any quilt I have made. I also have used fabric of any kind, old sheets, clothes for strip quilts or foundation blocks. When I made watercolor quilts I couldn't find many floral prints and used cotton blouses, etc. from our women's shelter. I have also bought wool skirts for wool applique, and penny rugs.
Recently when working on an applique I found I didn't like the flowers drawn on the back side. I ended up putting it on used dryer sheets, and plan on putting on the quilt, after I cut close to the work, I will whip it down so that the fine dryer sheet won't show. (I hope!)
Our ancestors made quilts because they were frugal. Most quilts were fabrics that the kids had grown out of,(or they had torn, etc.) and there were some pieces left, that could be used. {Early settlers had to spin cotton, then weave it into fabric. Can you imagine with the labor that was how very frugal they were with fabric?} Many times those fabrics were looked at by family members who remembered the person wearing the shirt, or dress, long after they were gone. Like a patched scrapbook of memories.