The October 2012 issue of Airmail, the monthly Airheads Beemer Club magazine, has an ad for MotoKeys.
http://motokeys.com/They offer "...keys to fit Airheads: OEM BMW folders, Neiman/GKS, brass or steel."
The Motokeys website also provides a technical drawing of each key so you can put your vernier caliper to work and confirm the correct size prior to ordering. That's a really nice feature, IMO.
Motokeys also has key blanks for later model BMW motorcycles and offers to cut Airhead keys by code.
I need to order a couple to cut as spares for two of my Airheads.It's kinda scary having only
one key.