In Munich you can take a look at a company called Mr Lodge. They are not cheap but that's a healthy budget that will get you a fully furnished one bedroom in the most desirable parts of town.

You'd definitely find students willing to rent out their rooms for the summer, but something tells me if you're ready to spend that kind of money you're over the student experience at any rate try wg-gesucht or one of the numerous Facebook groups. If someone asks for money before showing you the apartment, it's a scam.