My assumpting is that most 36" fixtures have a 96W light. I would personally stick with that same wattage of light if for no other reason than to max your lighting.
However I will be the FIRST in line to admit that I am not a lighting expert - I suspect the ballast would fire a 55W or 65W (shorter bulbs typically) as well. The issue there could be securing the lights in the fixture, assuming there is some sort of bulb clip. But, IMO, you are giving up some "juice" in the lighting. Considering you don't really need to replace them that often it is not a huge deal I think. Do some online searches, BTW, very much available online (I was searching today actually).