Ace Hardware, Napa Auto, etc.
I typically use 5/8-11
A 3/4 hole saw will give you a pretty good tennon on the tip of the horn to work the die onto. It will take just a tiny bit of file work for a good fit.
For a smaller horn, a 1/2" coarse thread is a good size.
You can PM me with any questions you might have.
A question I have is would a carbide die be worth the money?
John (Bigsmoke)