Do you have any other chucks or ways to hold material? 3-jaw, centres with dog, etc? How about thread pitch gauge, or the ability to carefully measure things with calipers? Can you grind a HSS tool blank to match a shape?
With the above and a few skills, this isn't too difficult a thing to make yourself. Even if it isn't perfect you can get by until you develop more skills to make a better one. It's a great learning project with real purpose. Make a few spares while you're set up.
Alternatively if threading is not your cup of tea, perhaps you can find some threaded rod with the correct thread type and pitch and modify it to suit. The recessed square typical of most chucks isn't necessary. You could do it a number of ways that are easier to make.
Finally, is there someone else that would like a moderately challenging project near you?