You should be able to get those rubber grommets at a lot of electronic supply houses...
The bulb should be pretty easily available too... but I'd recommend cleaning the contacts and using a good dielectric grease when putting the new bulb in...it'll help prevent corrosion.
Thanks sailingdog. Any idea how you get the bulb out? I tried the push and rotate method, but I think I'm supposed to release it--maybe with a screwdriver on the side? Not sure if you're familiar with this fixture or not (new to me).
Also, do you think Defender would have the parts and grease? Just asking because my boat's not far from there.