Well, I spent most of yesterday visiting half a dozen truck parts places, several heavy equipment outfits, 4 ag/compact tractor dealers, and 3 different auto parts chains. Nobody had anything that even remotely resembles what I need. I ended up buying a gas-charged strut that's the right length, but is way too stiff for my needs, I think. We'll see.
I think my only choice is going to be to find a manufacturer of these things and match up something, or buy a whole new seat. I found a company, called Enidine I think, that makes exactly what I'm looking for. They call it a "rate controller", and it's adjustable for different compression and tension settings. Unfortunately, they make them too big and too small, or close to the right size with completely wrong ends. So, I may have to buy something close, cut the ends off, and weld the correct ones back on. So much for the warranty...