Hi Ian,
Long time since I have been able to post on MLU due to other projects and work.
You might use eBay USA, and search for
Boat Horn Surprisingly they were used on boats too.
I purchased a second one for my F15A some years back for its mesh screen and undamaged rear. Be aware that they're 6V and sound like a wounded duck on 12V power.
There is also a 1 microfarad (often lost) capacitor which suppresses the AM noise from the horn's oscillation. This can be omitted in the 21st century as we mainly use FM and UHF which does not get interference.
When you find a horn it is a good idea to dismantle it and clean the spring metal disc and then lubricate it so the horn will work correctly. Mark all the joins so that you assemble it correctly or they won't work. Make sure all the screws are tightened evenly too.
PS Photo #3 is of a GM (Delco Remy) horn. As used on Land Rovers and 50's Holdens.
Cheers,
Ian (Williams)