I've never tried replacing one of these, but if you can remove the bulb from the switch, identify it and find it's not readily available, try calling:

Mica Company
5960 A Deale-Churchton Road
Deale MD 20751
800-333-5292

I've used them for many years as a source for obscure replacement lamps for electronic test equipment, television and radio production equipment, etc. If there isn't an identifying number on the lamp, you might find voltage, current and wattage rating in a Haynes manual by finding the switch in a schematic diagram.

Lamps are like bearings in most cases in that for the most part, they're standard items manufacturers source elsewhere. (In this case it would be the switch manufacturer.)

Mica Co. is very reasonably priced in my experience and if you want to play detective you might save a few bucks. Let us know what you find out.

Good luck,
--
Erwin in Memphis, '95855t, '86 Guzzi LeMans, '76 Montesa 348 Cota