The best known library database devoted solely to dissertations is produced by ProQuest. The database is split into two pieces: one for Humanities & Social Sciences, and another for Sciences & Engineering. This split reflects the legacy of the old print version of the product, also known as Dissertations Abstracts International.
Pfau Library has access only to the Humanities and Social Sciences portion.
Full text for many disserations is included in the database, particularly from the mid-1980's to the present.
Many dissertations and theses can be obtained through Interlibrary Loan; request them as if they were books.
Some libraries will not loan their theses. In that case, your option is to check for an Open Access copy, or buy a personal copy from ProQuest/UMI.
Unless they are Open Access, master's theses can be extremely difficult to find. Although there are some in ProQuest's Dissertations & Theses database, most schools submit only their PhD level work to ProQuest.
Many subject-specific research databases include references to dissertations, as well as journals and book chapters.