Try first to look in Google Scholar and Dissertations & Theses Global Full Text (above) for digital fulltext of a dissertation. Many universities are now posting dissertations in repositories (like Digital Commons). Google Scholar includes Digital Commons and other repositories in its search. If that doesn't work, you might also try a regular Google search.
An effective way to search for the dissertation in Google Scholar or Google is to place the title in quotes, followed by the author last name. Example search: "Self-Efficacy and Instructional Leadership" Helber
For older dissertations, you may need to try to interlibrary loan the dissertation. You can get an idea of whether the dissertation is at a library by searching OCLC WorldCat. Or go to the web site for the library of the university where the author did the dissertation. Unfortunately, sometimes dissertations are not available via interlibrary loan. As a last resort, you may be able to purchase dissertations from ProQuest--see DissertationExpress link below.