Whenthe light went out on my 97 540i I was distressed to find they use an H7 65w bulb for low beams. I went to a local AZ and a Discount Aut store and neither had it on a Friday night. The next day I found one at the Carquest because the dealer parts dept was closed.
It sounds like you need a H4 bulb which is about as common as it gets. All of the stores would have had it. Finding one in those wattages will be another story. The standard H4 is 55/60w and should be less than 5 bucks at those stores. I would be very carefull how much driving one does using 100w with standard wiring. (in other words a duty cycle or more than 20% - my guess).
Bosch Master, ASE Master, L1
33 years MB technician