There are several different styles of 9x12cm plate holders, and any one camera can only use one style of holders. The most common is called "Universalfalz", and was used on (among others) all Voigtländer plate cameras. I believe these are compatible with the Maximar, but can't promise anything.

Film inserts show up on ebay occasionally, especially on ebay.de (German Ebay). But most of them come complete with plate holders and associated camera, which of course is as good an excuse as any to building up a small collection of old 9x12cm plate cameras.