I had a similar issue when preparing for case interviews in Italian - while I am fluent, I had never used Italian in a business context, so my vocabulary in certain things was lacking.
To find cases in German, you could try contact German/Austrian Business Schools. Business Schools usually make their own case books so this is where you are most likely to find resources already in German.
For building vocabulary, suggest that as you go through cases in English, make a list of the words you typically use (e.g. revenue, profits, market share, etc.) and the German translation.
Preplounge has a strong German community, so I am sure that if you try, you can find someone willing to practice with you in German, and maybe even provide you with their own German case resources. Alternatively, Sidi, another expert on this site, is German and so he might be able to help you through scheduling a session with him.