Here is the part number: 600-6650-047 Diaphragm/ rubber (pw part #6650-47).
I checked a couple of parts dealers, they don't have any.
Update: According to Lionel, NL Trainworks has them.
If you take yours apart, you may be able to revive the one that is there by a careful cleaning with warm soapy water.
Sometimes if you work the mechanism a bunch of times it gets better.
(I've done both)
Note: above I mentioned that air has to get in and out. I think there is a paper gasket between the rubber diaphragm and the plastic launch mechanism body. Try not to damage it. It may be made from blotter paper.