I would say Tempelhof Ikea would be best location but there are other options apart from Ikea, chains like Poco Domäne (one in Kreuzberg: poco.de/nc/markt/standort/berlin-kreuzberg/
and Möbel Höffner (that was where I bought a folding table and chair for our flat, one branch is near(ish) Pankstrasse U-Bahn, on U-8. which goes down to kreuzberg. www.hoeffner.de
Note that you will almost certainly have to pay cash, assuming you don't have a German-issued EC Card. Ikea Germany doesn't accept credit-cards (I've double-checked on the website as someone had posted here that some branches did), nor does Höffner. poco will almost certainly be the same.
I have carried stuff from Höffner 8incl. a chair for the balcony) on the U-Bahn and it is Ok if you avoid peak times. Buses are not so good!
Edited: 4:00 pm, August 05, 2012