Interlibrary loans (ILL) provides access to journal articles, books and other items unavailable through the library. The service is available to all students, staff and faculty, is usually free (although not in the case of most dissertations) and can take anywhere from a few days to a few weeks, depending on the library supplying us.
For more information, see the Interlibrary Loan page.
A dissertation is definied as a lengthy, formal treatise, especially one written by a candidate for the doctoral degree at a university. Sometimes these are indexed in subject databases. Many times they aren't, so there are specialized indexes for them. A few of those specialized indexes are below. ProQuest Dissertation & Theses Global (at the top of the list) is an excellent place to look.
If you need a dissertation found in one of these indexes and the dissertation can't be found online, then it's going to take the library's time and probably your money to obtain the dissertation. The people to talk to are the interlibrary loans staff - see the box to the left. The good news is that many universities are making an effort to put their student's dissertations online.