They are hard to find in the lengths needed to live thread a shaft.
Chris Hightower has this threading jig for his lathes: https://www.cuesmith.com/thread-milling-attachment.html
There is a drop down toward the bottom of the order form for extra bits. They are 1/8" shank. I don't know if he still sells them separately or not.
Good luck :thumbup:
Gary