There are a number of databases and websites that allow you access to citations or fulltext of theses:
Subscription Database (requires login):
ProQuest Dissertations & Theses Global (PQDT)
This database is a comprehensive collection of international research postgraduate theses with fulltext from 1997. Check out the PQDT LibGuide for hints and tips on searching the database for particular theses.
Center for Research Libraries
This site allows you to search for 800,000 doctoral dissertations from universities outside of the United States and Canada.
DART-Europe E-theses Portal
This portal allows access to open access research theses from 535 universities sourced from 27 European countries.
EThOS (Electronic Theses Online System)
This database lists 250,000+ records for theses from participating UK Higher Education Institutions.
Networked Digital Library of Theses and Dissertations (NDLTD)
A digital library of electronic theses and dissertations (ETDs) from selected universities.
Includes more than 25 million records representing a wide variety of open access digital resources worldwide, including theses.