Not saying they still supply it, but this is the type of thing Motormite/Dorman (aka HELP rack at your parts store) may well have cataloged. There is also a Chinese importer that has gotten into reproduction of vintage turn signal switches big time.
You might also try these guys:
http://www.shee-mar.com/start.html
They don't show the specific year application, but that does not necessarily mean they don't actually have what you need, only that they don't catalog it that way.