that's a toughy Karli...All the used clothing stores I know are clearly by donation, or esle they're really expensive and I have a feeling they buy it from somewhere. You could try some of them though.
Also, I haven't seen a lot of used clothing stores here...people are really into the 'vintage thing', especially the 'ugly polyester vintage' thing.
BUT I don't know everything. I've mostly only kicked around Mitte, Prenzlauer Berg,
Kreuzberg and Charlottenburg, and there's a whole lot of city outside that!