H-German's guide to German Archives.
The page notes that it is not complete; however it is a good starting point.
More up-to-date is "Travailler dans les archives en Allemagne" -- the important parts are in German. Or ask me.
The Austrian Dissertationendatenbank & Deutsche Nationalbibliothek collects online dissertations (Hochschulschriften) : how to search for them is explained in German.
Search across more than 424 universites in 23 European countries through the DART-Europe E-theses Portal.
There's a good guide, "Trouver des thèses en Allemagne," that covers finding theses from the pre-electronic era -- the important parts are in German. Or ask me.