The original, South American variant, is a tea-like infusion, which is supposed to be bitter and sugar-free :) (though there's a "mate dulce" sweet variant)

http://en.wikipedia.org/wiki/Mate_(beverage)

I know it's sold in Austria, but I don't know whether you can find it in Germany.

Edit: apparently it's sold at speciality shops in Germany too, like Delicatino

www.delicatino.de/es/nosotros?b98bf9d568d475199bf586135367cc54=qjqxosww

or

http://www.yerba-mate.de/

http://www.gustoargentino.com/

http://www.bockandgardener.com/

Even the typical Uruguayan variant is available at stores like Mittemeer and Aquí España:

http://tropicalizandoberlin.com/?tag=yerba-mate-en-berlin-2