There's a guy near Beirut in Joünié, Lebanon (also spelled Jounieh more commonly in English) who reproduces them (EDIT - same eBay seller that I see SpruceItUp linked above!). He sells them on eBay on and off, Malik L Crosset aka George Loussian. He was North of Beirut, I do recall that. Lebanon is a warzone currently though, so this may be a bad time to hope for getting any parts from there, so you may need to just keep an eye out on eBay. He has eBay auctions up currently, including a used shaft, and new versions, he is a machinist.
https://www.ebay.com/str/georgemotopart
He has them used, new reproduction, and new reproduction with the left hand thread nut for $15 additional.
https://www.ebay.com/itm/355341766574
https://www.ebay.com/itm/358441301119
Here's another repro, older post, not sure if they are still doing them:
Another one, recent, one-off for a customer's bike:
Or try this guy, he has made several different KX versions of this shaft and certainly could do a one-off. Czek Republic or somewhere I believe, so also more shipping and tarriffs to add to the cost.
kxpowervalve.com