Hi Lellu,
there is a pretty good CD store on the first floor (2nd in the US) above one of the exits at Zoo Station on the U2 ubahn line. There is a plaza there, and the CD store is at the same end of the plaza as the zoo, but across the plaza. I bought several CDs for the Moon when I was there a little over a year ago
If you can find this somewhat famous church that was damaged in WWII, when you see this view, turn around and the CD shop will be almost directly in front of you
Happy hunting.