They're around.....
You likely have a Yacht Specialties/Merriman wheel assembly. I see these units, on a regular-but-not-scheduled basis at marine used parts stores around the NW. Have you checked around your geographic region? We have a couple of stores in PDX and there are at least two large ones that I know of in Seattle.....
It might be cheaper to find a good base and column and re-combine the best parts out of several....
(Like the great scene in "The Good, Bad, and Ugly" where Tuco assembles the perfect revolver....
)
Or, get out your checkbook and convert to a new Whitstock or Edson or similar...
Best,
Loren