A couple things to try first; (1) switch the left bulb to the right and vice versa and (2) detach the plugs and reattach again to ensure a good connection. Doing (1) is probably good enough. Maybe you could switch the wires too. I've had this problem a couple times on two different 5 series and this trick worked both times. Finally, it's very possible that one bulb is not as good as the other. Just buy a new pair so that they match.
The dealer in sweden only has the "housing" and the bulb as one unit.
But from what i can tell its possible to get just a bulb at any autostore, the hard part is to get the bulb out of the housing
This is what the part looks like,