There are a number of databases and websites that allow you access to citations or full text of theses:
Subscription Database (requires login):
ProQuest Dissertations & Theses Global (PQDT)
This database is a comprehensive collection of international research postgraduate theses from 1743, with full text added to theses 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 span.
DART-Europe E-theses Portal
This portal allows access to 412196 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.
PhdData: The Universal Index of Dissertations in Progress
This database has citations from several thousand dissertations in progress from various parts of the world.