There are publication requirements for theses and postdoctoral theses. Therefore you will find them like books in library catalogues (comprehensive search) and you can order them via Interlibrary loan.
Theses are published more and more online. Various databases offer a comprehensive search, e.g. DART-Europe E-theses Portal.
There is no requirement to publish examination papers and study theses (Bachelor, Master, Diplom). Therefore, they are rarely available in German libraries. Please ask the chair of the university where the study thesis has been submitted.