What bike is your new/old rim from? Using that information you can find out what bearing BMW specified, which includes the size in mm, and probably source a sealed bearing replacement from any bearing supply store. I don't know what the guy thought was complicated about the BMW "system." I understand BMW went to sealed bearings later on, (in the LSs too?) but I don't know if they were the same size as the older type.
Of course, where will your replacement bearings have been made?

So your wheel doesn't have the pre-load setting shims, either?