A number of electronic databases include dissertations and theses in addition to journal articles, books, chapters, Festschrifts and other scholarly materials. Among them are:
Center For Research Libraries - CRL holds more than 800,000 doctoral dissertations outside of the U.S. and Canada. Search dissertations in the dissertations section of the CRL catalogue. Digitized dissertations can be searched in the catalogue's e-resources section.
Networked Digital Library of Theses and Dissertations - A collection of more than 800,000 international full text theses and dissertations.
OpenDOAR - The Directory of Open Access Repositories - Provides a quality-assured listing of open access repositories around the world.
PhdData - An open database of doctoral dissertations in progress around the world.
Google Scholar is primarily a tool for searching for articles, thus it includes few full-text dissertations or theses (this largely depends on how widely an author wants their work disseminated). However, Google Scholar may be a good place to look for references to dissertations which may appear in article bibliographies. If these references are to unpublished works, it may be difficult to track down the full document unless you contact the author directly.