Temporarily, you might use the fitting for the holding tank deck pumpout. It should be the same size thread and it won't do any harm to have the holding tank uncovered (in fact, the extra ventilation will probably help to reduce the odors). Of course, that fitting is labelled "WASTE", so don't let any dock attendants get confused.
These fittings are pretty standard, so you should be able to find a replacement easily. Jamestown (in Camaraderie's link above) is a good place to start, but you may need to bring one of the fittings with you to a chandlery to make sure you get the correct thread size.
Yes, PSC seems to be on the ropes, unfortunately. We probably won't get much help from the company anymore. But you might consider subscribing to Sailnet's PSC e-mail list-serve, which is frequented by many more owners than this forum. See: http://list.sailnet.net/read/?forum=pacificseacraft