Phone audio stores, and they should be able to help you with the parts.
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This is a year later but maybe you still need an answer. I just encountered the same problem with my HD540's. Replacement is simple: just pull the ear cushion and it will pop off the headphone. It's pressure fit holds the thin foam in place, so when you pop-off the cushion, the thin foam comes lose. You can either cut a new piece of foam or use a piece of non-sound-absorptive material like thin nylon or Cool Max (the stuff running apparel is made of). I cut some pieces from an old running shirt and it works fine...looks a heck of a lot better than the foam, too.
Jim in Dallas TX