There was a pretty good International Bookshop (I think that was the name, even) in Berlin but of course that's not too close to Tübingen. A bit older books were quite affordable there.
For Internet, one likes to recommend
http://www.play.comIt's not the cheapest around necessarily, nor yet does it have the largest stock list in the virtual 'verse, but it's postage free and the books arrive quickly and singly in inconspicious brown packages...

There's also a used books market called PlayTrade where you can even get new books for cheaper than the actual site has them.