I had a feeling that lamp was an LED so I checked the manual and it's this one:
http://www.marcospecialties.com/pinball-parts/112-5033-08
you can use any 6v #555 pinball LED from cointaker, comet, marco, etc. just picking marco since they had the product number.
Quoted from LOTR_breath:Bulbs go out for no reason at all. You can even have a bad bulb right out of a new box. Every pinball owner should have a supply of #555 and #44 bulbs on hand. I buy the boxes of 100 from PBL to save money.