Try this Yamaha USA parts number: 888-892-6242 (option #1). They have been a source for replacement P-32D reed plates. I don’t know whether they would have gaskets. There’s a delivery charge, but that can be avoided if a local authorized Yamaha musical instruments dealer will order for you.
Alternatively, you can create your own gasket. I just ordered the following, which would probably work:
http://www.parts-express.com/parts-express-speaker-gasketing-tape-1-8-x-3-8-x-50-ft-roll–260-540
It would have to be cut into 1/8″ strips and glued at the joint(s). If you are still in need of a gasket after I get it, I’ll be happy to send you some of this.