Beer steins in Berlin

You can find beer steins all over Germany, including Berlin. What do you consider "authentic"?

You might want to try the stores on Unter den Linden in front of Pariser Platz that sell Berlin tourist items.

Well, you are in Berlin, Germany so most likely it is going to be authentic German made...unless you turn it over and it says "Made in China".

Stoneware tankards or "steins" are a Bavarian thing, not a German one.

So finding an "authentic German stein in Berlin" is a little bit like finding an "authentic American Hawai'i shirt in NYC."

That said finding such a think is of course not impossible, depending on what you would accept as "authentic." I would ask in normal department or tableware stores if they have simple Keferloher tankards:
http://de.wikipedia.org/wiki/Keferloher

Authentic beer vessels from Berlin would be Berliner Weiße glasses or Bierkugel glasses.