Where can I obtain official German VFR approach charts?
The official German AIP VFR is only distributed via a company called Eisenschmidt. I'm not even sure, there's a digital version of the official charts. But you can buy a 2.35kg heavy print version of the VFR AIP. The only thing that comes close to the charts digitally is something called "VFR eBulletin", a software, where you can enter your route and see that overlaying an ICAO map - but the map is in very bad resolution and low quality. 

Sadly, Germany doesn't do it like many other European countries by publishing their AIP VFR online for free.