The problem is that 6.5x9 is a Eurpoean size, and it's not comatible with US/Imperial sizes. There is no problem finding 6.5x9cm over here (MACO Genius is very easy, from www.monochrom.com, website in German only)!

The Avus is a lovely little camera, and I'm glad to hear you want to use it. I've just been out with the "big brother", a Bergheil 9x12cm. I use it for "street photography"! Nobody seems to believe I can take sharp pictures with that ancient thing, so in a way it's even more unobtrusive than a Leica. But the sharpness is astounding...