This is a very good database and one of our best resources for identifying theses and dissertations worldwide.
Their claim: ProQuest Dissertations & Theses Global is the world's most comprehensive collection of dissertations and theses from around the world, spanning from 1743 to the present day.
If you are not sure about all the details of the thesis or dissertation you are looking for, you can use Proquest's Advanced Search to look for author, advisor, university or institution, subject or keyword, language, etc.