This appears to be the kind of bullet-end bulb that you find in the overhead light fixture in your car. It takes a double-end mount, and I've seen two kinds; most prevalent is the sort that it just clips into -- the mount blades have indentations that hold the bulb in place and provide power. One end is hot, and the other grounds to the vehicle. You could also use a standard "fuse" type mount, but that doesn't provide fixed lateral positioning.
I suspect you could find these -- cheap -- in the auto graveyard, or possibly at an auto parts store by buying the whole overhead light fixture.