Clyde (lt1) sells them. I don't see a link for them. You'll have to just send him a PM through the site. I don't see where he accepts emails from the site. You can locate him through the "members" button on the top bar.
Also, I had seen that Clyde posted in the " BMW K1600GTL vs C14" thread in the open forum. Here's a link to the thread. Clyde (lt1) is post #4.
http://www.zggtr.org/index.php?topic=1383.0His signature says: "Still have a few mirror risers in stock."