Guten Tag my german friend and welcome to the community 
You might want to try this thread (in english) if you want to find help on building your own fairphoneOS. You might find someone kind enough to provide you with links to his version of the fairphoneOS by PM (please note that it’s forbidden to share ROMs for the moment. Fairphone said they were working on solutions three weeks ago but we don’t have news about it since.
@Irina_Spitznagel’s link worked well for me too. Just be aware that you can’t have a fully Google-free phone with a Gapps-tainted ROM (because google apps replace some of Android’s functionalities).
If you feel like it, you might start a german thread for building the OS ! I suggest you emulate a GNU/Linux distro with virtualbox if you want to build fairphoneOS.