Dennis, your suggestion of Sydney, now Vector Marine in BC was a great one! I'll add my plug for them here too. They've been very helpful and responsive and seem a great bunch of folks.
They even have a used A30 flexible shaft coupler they're willing to sell me! So that's awesome! It's the first source I've found for these.
The only downside, is that since they won't be involved in the installation. they can't give me any warranty for it. I'm worried, being a used part, that it is one gear shift away from failing; that I'll install it, and the the metal will shear from the rubber on the first shift, being probably as old as the my original one that failed in that way. They sent pics of it and it seems in good shape, but how could you tell.
Since the part isn't cheap, that's a gamble!
On the other hand, there doesn't seem to be any alternative. I haven't found any other source for this part, new or used. It seems unlikely that I'll find a new one, and no one is likely to give me a warranty on a used one.