Any commercial speedometer shop should have them as the Argo units are still in use and serviced.
Here's what one of two mounted in the mechanism case look like.
To change bulbs; open the case as you would to change paper recorder discs (under the discs are the bulb holders) pull the long spring-loaded tab away from the bulb center contact and the bulb should come out with a minimum of effort.
If you don't have a key, the speedometer shop can help you with that too.
Bonehead question: You DO have power to the bulb sockets, right?