"Why not just use felt? It might be a little heavier to get adequate filtration, but it could work fine in a pinch. It's also a non woven fabric. But the big thing about those machines is their ability to make of amounts of cheap fabric so it's cost effective as a DISPOSABLE item.  But once their cost skyrockets or especially if it doesn't meet demand in a crisis. Then many other materials become viable & maybe even competitive, at least in the short term."