Mannheim isn't near me, I'm way north of you. I rarely eat at restaurants that serve wild game. Though I have sold to more than a few. No law against selling wild game in Germany.
Best way is to get in contact with a local Hunting club. Usually pretty conservative types, not often the warm and fuzzy types. Tell them what you want. The closer you buy to the source the cheaper it is going to be. Price doubles every time it changes hands. 9 Euro's on the low side 16 Euro's on the high side, per kilogram direct from the hunter (or Forstamt). The Forstamt is also a good place to ask, they manage the city/state forests. The price at the Forstamt is stable depending on how much you buy, but availability varies. In restaurants it costs more than Porterhouse steak. There are Butchers who specialize in wild game, a likely place to ask questions. Search
mannheim wildfleisch metzgerei
A Metzgerei or the Forstamt tests for radioactivity. Which is still a concern after Chernobyl.
I cook it just like Pork, slow coke covered for a long time, keep moist. I mostly make Goulash (γκούλας) slow cook for hours, add potatoes after the meat is fork soft, add vegetables just before serving. The Greek are famous for their Pork dishes.