I believe it's 'bituminous' disks, anyway, a great substitute that does the job (better, I think) are the thin, nickel-sized clear vinyl disks you can buy in packs at Ace Hardware or HD (meant for putting under glass tops on wood furniture). As long as they're clean and the surfaces are clean, they'll 'stick' the two units together and they don't compress (i.e. allow the Watt to 'bounce' on the Puppy.)
A side benefit is they won't mar the speaker finish, which if I remember correctly, the original disks could do.
A side benefit is they won't mar the speaker finish, which if I remember correctly, the original disks could do.