I go to garage sales and find mattress covers, also use old quilts (not mine LOL), and for a heavier quilt, I use denim.
In the old days they put old denim from jeans in the quilts for warmth. ON a budget..I have also pieced together those side pieces you get when you cut your size. Use a zigzag stitch. WOrks well. Nothing wasted on thsi end.lol. Project Linus goes to a batting store and buys it buy the roll. A lot of guild members here do too. Cheaper, like half as much. Happy Quilting