They have them at the asian stores. I usually buy mine at the one in
hauptbahnhof, just next to the U2 exit. If they don't have them there then try the large asian shop in
Rosenheimerplatz, next to the hit. If it still doesn't work out then go to the other large asian shop in Hohenzollerplatz.
I'm going out right now to one of those to buy some rice flour and food coloring for some Thai dessert I'm preparing later. Will let you know here if I see plantains.
Edit: it may also be that you have to buy them still green and have them at home for a couple of days. They don't always have the ripe ones.