I would check out the thrifty stores for flat sheets, but make sure they don't have a high polyester content. That's what parkas are made of [and probably parachutes], and it doesn't sew well at all. All of my quilt backings are done with inexpensive sheets I've bought when on sale, only had 20/40% poly, and they have sewn up well. If they seem a bit soft to handle then starch them well first.
You will probably find polyester wadding is the cheapest to buy. It might be a bit difficult to handle in a domestic machine, but you can always cut it into managable strips and pin and quilt that, then add another strip and repeat till done.
Hugs