We are in the same boat Randall. We have a '78 Sportsman, same engine and tranny. We have searched for 3 years and had the same bad luck and we currently use our GPS as a speedometer. If you ever find a source, we will be glad to hear about it! Good Luck!
I had same problem with a 79 sportsman. Rv salvage yard even laughed and told me good luck on new replacement parts. I ended up bypassing the cruise and was lucky enough to scavange a cable to make speedo work,(somewhat). it would sporactically jump around when it felt like it. Don't know why they designed something that they assume won't need replacing in the future. I checked with all my contacts and they say the same thing,, good luck finding something for replacement.
If you still have your old cable pieces you should be able to take them to an International truck parts store and find some help. They usually have kits to assemble any cable from parts in stock. Just a matter of matching up ends.