My dealer told me that the following is an emergency spare for the 7:
Emergency Rim 36116796147: 18x4.5
Tire: 135 80r18 Conti
All in cost including jack kit is about $569.
For me the question is whether it is small/light enough to make it worth buying instead of one of the Alpina 18" winter wheels that I will have anyway.
Apparently it has to special ordered from Germany. Maybe that is why you can't find it.