UK is closer than US!
Mamiya agents are Johnsons Photopia:
My favorite independent repairers (although I know several good ones) are Newton Ellis & Co in Liverpool:
Hope this is some help!
Regards (or as you're in Stuttgart - Grüß Gottle!),
PS: In my previous reply, I of course meant focusing ring/mechanism instead of aperture ring/mechanism. The repairman recommendations still apply!