Being one to try as I might to buy American (sans HD) whenever I can, here is a list of Yankee Vendors making Farkles for your German Bike.
BoxerMetal, Chris makes all kinds of Custom Airhead parts like airbox eliminators and Acewell Speedo Dashboards.
Twin Headlight Ernie, Maker of the strongest toughest badass Bash Plate that you could ever hang off your GS.
Jesse Luggage, Just did a special batch of Old School Bags for the R100GS
Hardwaregirl, check spellin, this little lady makes a Killer GS Solo Seat Rack.
Beemer Buddies, Handgrip Wraps.
HPMGuy, Top Triple for Airhead GS
Toaster Tan, Top Triple for Airhead GS
Flip-A-Lever, New and Improved Throttle Screw.
Feel Free to add more and Thank You for Buying American