I appears you need a G3.5 or G3 1/2 bulb with a bayonet base. It is also referred to as a mini indicator bulb. Looking on amazon, there are still some companies that make them.
If you want to go to the parts store (or radio shack) and look, your specs are:
~3/8" diameter base
~15/16" length
bayonet base
10 pack on amazon: https://www.amazon.com/Eiko-Indicator-0-135-Miniature-Bayonet/dp/B003R3LAQA/ref=sr_1_7?ie=UTF8&qid=1476980153&sr=8-7&keywords=G3.5+bulb+bayonet
Some that look close:
SYLVANIA 53
SYLVANIA 57
GE Lighting 53
GE Lighting 25550
You should be able to find a '53' or '57' bulb at most auto parts stores.