Originally Posted by
cph_flyer
Get it at the airport .There is a ticket office right by the bus stops at TXL.
The linked map shows the location of the BVG ticket office referenced by
cph_flyer:
http://www.berlin-airport.de/EN/Reis...DownloadEN.gif
Once you have your ticket or pass in hand, you exit via the adjacent door to the area where the bus stops are located.