You will probably have to cut those threads on a lathe.
You're asking for a "tap," and not a "die," so I am assuming that you want to put threads into a hole, for instance in a receiver or frame, rather than onto a barrel.
The lathe setup to do this is difficult, and it is not something to be attempted by the faint-of-heart. I suggest that you need to consult an extremely experienced machinist.
(There might exist such a thing as a 1/2"-32 tap, and that might be close enough. You would probably want an interference fit anyway, if you're seating a barrel. Consult a machinist-supply house for that size of tap. You should be able to find one on the web.)