Dissertations and Theses are considered grey literature. These resources can be found in many different places: Databases, websites, catalogs and WorldCat (combination of catalog/database).
Dissertations and Theses Resources:
OATD.org aims to be the best possible resource for finding open access graduate theses and dissertations published around the world. Metadata (information about the theses) comes from over 1100 colleges, universities, and research institutions. OATD currently indexes 3,897,764 theses and dissertations.
OCLC's WorldCat lists dissertations for most colleges and universities in the United States and Canada and a great many from around the world.
The free version of WorldCat can be found here.