Hornig, Wunderlich, and I search German internet also. There are used parts dealers in Germany that ship. I bought a set of camshafts for my K13S that way. Of course there is EBay for used parts. The wunderlich catalog is incorrect for front wheel bearings for K13S. For specific parts for the bike OEM is your only source. Most aftermarket places have few specific parts. But a good example of savings is buying the front ball joints for K bikes from Wunderlich rather than BMW. Also Euromoto electric has lots of good stuff.
Living in Canada cuts you out of the loop to some extent. Some suppliers will not ship there from USA due to the hassle. I would also look at the UK for parts.