I like to use Wikipedia a lot for such things, and since I can read German, I can find a lot of things there. Note that the excellent images often included might disappear sometime soon - it was once thought that anything produced by the German government was automatically a part of the public domain, but last year there was a lawsuit regarding the copyright of some 2011 stamps containing content by Loriot, a famous humourist. It was decided by the courts that Wikipedia did not have the authority to publish the images, and they have been forbidden to show the stamps (but they can show his signature, for what that's worth).
The upshot of this is that the template used in German Wikipedia to establish their authority to reproduce these stamps is not universally correct in the legal sense, so there's a chance that they'll remove all stamp images if they aren't able to come up with legal justification of their right to show them. Image pages for German stamps now include the following text:
Quote:
This file is most likely NOT in the public domain. It has been marked for review, and will be deleted in due course if the review does not find it to be in the public domain.
This file was previously considered to be in the public domain as a German stamp; it is possible that it is in the public domain for other reasons, in which case a new rationale will be applied as part of the review process. See below for the previous rationale.
A bit of info on Loriot, with a section near the end on the lawsuit.
http://en.wikipedia.org/wiki/Vicco_von_B%C3%BClowRyan