I'm trying to mount a ham radio antenna that has a standard(?) 3/8-24 threaded post at the bottom to an existing 5/16" post (built in the past for a different antenna) with 5/16x18 threads.

In other words, I need an adapter that's metal & conductive with a female 5/16-18 cavity/connector/receiver (not sure about nomenclature) at one end, and a female OR male 3/8-24 threaded cavity/connector/receiver or rod at the other (I have a female-female 3/8-24 connector, so the gender of the adapted 3/8-24 end doesn't matter).

The big question: what is this called, and is this something likely to exist as a retail product a local store somewhere in South Florida is likely to sell, so I could get my hands on it quickly & not end up having to spend $5-15 shipping on what's probably a cheap part? If so, what kind of store would I go to? Auto parts? Plumbing supply?

From what I understand, 1/8" FIP/MIP pipe fittings might have 3/8-24 threading, but pipe-threading isn't quite like bolt-threading, because it's designed to seal liquids as well as fasten them together, so screwing an antenna into a "1/8" pipe fitting could/would chew up the threads on the antenna, or fail to secure enough of it to prevent it from tearing out.