In keeping my 1956 30 Johnson as original as I can I am swapping out the 12 volt battery for a 6 volt battery. That being said I need to convert the running lights to 6 volts. Where do you find 6 volt bulbs or for that matter LEDS. Which are not an original item for this boat, but it is nice to be seem at night.
I think most larger auto parts stores will have 6 volt bulbs or can get them quickly from their distributor. There are also lots of online bulb stores. Be wary of the pricing, though. 6 volt led bulbs are around also. Just search online.
6 Volt bulbs are available online for the vintage car market. The websites may get confusing, but if you scroll through enough stuff, you should be able to find what you need. Pay attention to the bulb base and,if necessary, the candlepower/birghtness.
Dave
A websearch will give you some companies to check out.