Your local BMW dealer will supply you with one, no doubt. OK I guess you may have to buy the while assembly but the choice is that or blown bulbs, it seems. You can always try to sell the left over pieces.... Who knows, a broken stem may be just what someone is looking for.
Failing that, try Motoworks or whatever they are called or any of the other recognised breakers of all things BMW, which may (or may not) save you some wedge. They have an advert / contact details right here on UKGSer, for which they have rewarded the site's owner with much gold.