I lived there for half a year. But I still visit quite often.
One of the bigger stores is
Staedler but there are tons of cheaper and smaller shops throughout Berlin.
If you just want a cheap bike, look for second hand shops or visit some of the weekly
Fahrrad Flohmarkt (flea markets).
Other than that there are a few bike builders in Berlin with some really stylish bikes such as
Schindelhauer.
Me? I just took my own bike from home since I was coming in by car anyway.
Keep in mind though that Potsdam and Berlin are two different cities with quite a bit of distance between them so you might not be visiting Berlin that often.
