There used to be a place called "Silverton Supply" and they were out of Toms River, NJ, but I think they changed their name.
You might be able to find them with a google search.
They carried all the older Mainship parts.
That was likely the Silverton (factory) parts dept in Toms River (oddly, separated from the actual factory, at the time), Silverton being a sister brand of Mainship.
The guru there (Jim) retired just before Luhrs Group went south... they moved a sorta-kinda parts department briefly to St. Augustine (but that guy had no real background, certainly not Jim's long memory), then back to Jersey... and then Marlow bought Mainship and Dr. Trocki and Egg Harbor Group bought Silverton.
Dunno the outcome on the Mainship side, but the EHG (especially Lloyd) is at least trying to support older Silverton owners as best they can.
It still may be worth a call to Lloyd at EHG, since the brands actually used the same parts suppliers for most stuff... and the rub rail material of the Mainship II might be pretty much the same stuff as one on the Silverton 34C of the same era. He may be able to track it down, supplier and part number maybe, if you can describe the dimensions of what you have. OTOH, it may also well be that somebody in the owners group (yahoo or somewhere) already knows the answer, whereas Lloyd would have to research it from scratch.
FWIW, the Mainship III we had was actually built by Silverton Marine in Marlboro, NJ, so prior to the Luhrs Group spin-off of Mainship into its own brand with its own factory.
-Chris