JD has a surprising selection of old parts still in stock, unfortunately, they tend to waaaay over charge for them, which is why they're still sitting on the shelves.
2nd - Not sure what you'd be worried about wearing out - the chain or the gear? Either way - I'd imagine that would take a few decades to become much of a concern.
Guess it depends on how much you use the baler. But if it's under a couple thousand a year, I don't think I'd worry too much about it, and just change out the chain if it's broken.