If i were you i wound rule out BMW replacements as they are 200% mark up on how much they really cost and around £110.00 a set. I would ask HIDS DIRECT for replacement bulbs as they are good quility and cheaper (also you can get higher Kelvin rating so you can have blue white 8000K tint ones instead of the yellow white 4300K original ones they come with. Heres a link
Welcome to HID-Direct, We’re your no1 U. K supplier of HID products
I would call them (you'll find the number in the "contact us" part) and explain what you need as they don't list them but more than likely can get them.
Joe