There are 2 types (depending on how old is your lamp)
Here is a lead on how to search for them:
OLD BULB: Osram Sylvania 31393 (50W)
NEW BULB: Bulbrite 650051 (50W)
Note that there are other manufacturers too. The important specifications are BASE TYPE and WATTAGE
NEW BULB: Bulbrite 650051 (50W)