Hi Doug, unfortunately unlike the later 9x12 holders which have the same dimensions as a 5x4 darkslide so fit an international back, there's no standard for pre War German 9x12's.
There are at least 2 different Zeiss types of holder so you need to be careful when buying them to ensure they fit your camera. Try searching on ebay.de (Germany) the login details are the same as ebay.ca, .com or .co.uk.
There are usually spare plate holders listed you need to use the right German term the cameras are plattenkameras. Rollex 6x9 roll film backs are also quite common, I have two, one may be spare, but won't know until I return to the UK in a couple of weeks.
It's possible that the russian 9x12 plate/film holders fit a Zeiss as they made a copy for quite a number of years.