Go to a store, craft, fabric or even Walmart and find the pillow inserts. (ask for help if you need it!) There will be stuffing material there as well. :)
Many mattresses are made with layers of foam. If you could salvage a few of these, you’d have plenty of foam. I’d spray them down with lysol and let them air-dry sufficiently to remove any odor. Try to avoid children’s mattresses!