This guide outlines ways of identifying theses and dissertations published in your discipline, and those that are currently in progress.
This video from the British Library outlines how PhD theses obtained via its EThOS service can be used in your research.
Example in the Harvard style (you might need to format your reference differently when using another referencing style - check your departmental handbook):
Droth, M.S. (2000). The statuette and the role of the ornamental in late nineteenth century sculpture. Ph.D. Thesis. Reading University: U.K.
If you need help with finding information, then please contact your Academic Liaison Librarian for advice.