I think you might have to visit a fabric store. They sell some pretty thin materials - almost the consistency that you'd find on the bottom of the box spring in your bedroom. This is about the same density and thickness of the original material that was in place from Grado in the beginning.
Just be careful going into such a store - there are many old ladies with a passion for sewing that might try to hit on you by striking up a conversation on sewing. Just buy what you need and bail out of there.
psssht. I'm gonna find myself a lady with a nice 401k plan. I'm gonna be hitting on them, not the other way around.