For the most comprehensive search for dissertations, use a research database. None of the databases provide the full-text of the dissertation,so once you have all the information for a specific item (author, title, institution, etc.), you can request it through Interlibrary Loan.
Dissertation and Theses
Dissertations and Theses Database includes digitized dissertations in a variety of subject areas including Art, Communications, Education, History, Linguistics, Literature, and Social Sciences.
Index of published and unpublished dissertations from around the world. This database does not provide abstracts, but does provide subject headings.
America History and Life
US and Canadian history. Choose Advanced search. Under "Document Type" select "dissertation"
Art Full Text
Indexes over 550 journals covering a wide variety of the arts (film to fashion, classic to cutting edge, ancient to modern) from the US and around the world.
World history except US and Canada. Choose Advanced search. In "Document Type" type "dissertation"
Linguistics and Language Behavior Abstracts (LLBA)
Literature. Go to advanced search. Under Limit to: Document Type Phrase, choose "Dissertation Abstract."
Psychology. Under "Publication Type" choose "Dissertation Abstract."
Music. Go to advanced search. Under Limit to: Document Type, choose "Dissertation."
Social Services Abstracts
Indexes Dissertation Abstracts International: A. Try using the term "dissertation" with your topic keywords.
Indexes Dissertation Abstracts International: A Try using the term "dissertation" with your topic keywords.
Women's Studies International
Indexes both Dissertation Abstracts International A and B. Try using the term "dissertation" with your topic keywords.