Check online:

If you are looking for a UVM dissertation, search ScholarWorks.

Otherwise, search ProQuest Dissertations & Theses. If that is unsuccessful, try Google Scholar and NDLTD.

Find out if the dissertation was published:

Some dissertations are subsequently published by academic presses. A quick way to find out is by searching WorldCat. Search by author because the title of the published book may be different from the title of the dissertation.

Request an interlibrary loan:

If you are unable to find the dissertation online, request a copy via interlibrary loan.