Here are the two major databases which contain worldwide dissertations and masters theses. If you find a citation to one and you can't find it online, try getting it from Interlibrary Loan.
This database is the premier database for discovering dissertations and masters theses worldwide. It aslo has links to many online dissertations and theses.
It is a Subject, Title, and Author guide to U.S. dissertations accepted at accredited institutions since 1861. .Selected Masters theses have been included since 1962. Includes citations for dissertations from British universities from 1988 to the present. Some European universities are represented as well. Abstracts are included for doctoral records from July 1980 to the present, and for masters theses from spring 1988 to the present. You can search by Department, School Name, Advisor Name, or Degree Type, etc.
WorldCat is a good database to use after you have tried the Dissertation and Masters Thesis database above. If you need to do a thorough search for existing dissertations or theses it can be useful, as well as if you are looking for ones in languages other than English.
Using the Advanced Search in WorldCat. After you search, you can limit to dissertations or theses by Format on the left-hand side:
If you find a citation for a dissertation or a thesis that you can't find at PSU or online, you can try to interlibrary loan it.
Just click on the link and fill out a request http://library.pdx.edu/services/interlibrary-loan-article-delivery/