Some of the larger department stores such as Wertheim
Steglitz, or even Obi, have an art supplies (small) section in their Schreibwaren (writing material) dept.
Also Zeichen-Center Ebeling, Trifftstr 39,13353 Bln,
Farben Kacza, Oranienstr 173,
Kreuzberg (they might only have pigments but it's a nice place to visit)
Ferdinand Braune Künstlerbedarf, Grünewaldstr 87,
Schöneberg (Kleistpark U-Bahn)
The largest is Boesner, Nunsdorfer Ring, miles from nowhere, but there's a bus ... also an online shop