You may know these but if not here they are:
Blue Moon Cycles in Norcross is a Vintage BMW shop that can help you out with parts if you need them in a hurry. http://www.bluemooncycle.com/
Boxerworks out here in Athens also does airheads. http://boxerworks.com/
BMW Motorcycles of Atlanta is the dealership. They're in Marietta and they carry a few airhead parts too. Not much, but some. The parts guy there rides an airhead GS so he'll speak your language. http://www.bmwmcoa.com/
Those are your only 3 local resources that I know of. If they don't have it you're stuck ordering it.