From the Haynes manual, these appear to be tapered bearings. Industry part number should be stamped on one face. Not even BMW has enough volume to get a custom bearing for a motorcycle.
Often times, the metric tapers were sold as a set for precision tolerances. However, in a steering head bearing, no precision is needed (at least on the order of 'bearing' precision). Get the cup (outer race) part number off the rear rib and call a local bearing house. Motion Industries, King Bearing, etc. They should be able to help you out. It will be the same thing with the same dimensions. You may still need to purchase the complete bearing, but it should be less expensive than through BMW.
SKF, FAG, Timken, NSK, NTN, Koyo are all good brands. I would stay away from the Chinese (Peer).
Not only have I stayed in a Holiday Inn Express - I used to be a bearing engineer for Timken.