Years ago a forum member had a few 3 fin ones made. I purchased one for an arm and leg and still haven't mounted it The fitment is not 100%, but it's not bad. I believe your only option today is: http://www.goeckel.de/
You had better brush up on your German though..
The only other option would be to get a sponsor/vendor to create some if there was enough interest..
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.