I'm looking for the manga for Densha Otoko ("Train Man") series, but all I've found are chapters 1- 18 (until the end of Volume 2).
I've checked several manga sites for the continuation of the series, but can't seem to find anything worth downloading (in other words, I can only find the chapters I already have).
Any suggestions?

We have a thread for manga downloads. Check the sites listed there and ask in that thread if you can't find it.