To find dissertations published here at Cornell and elsewhere:
ProQuest has the UMI collection of dissertations from more than 700 institutions, published 1996-present. Members of the Cornell community have free access to PDF versions of many of the dissertations.
Cornell has many dissertations online via eCommons
Cornell's dissertations are also available in person. Go to the catalog and filter the format to Thesis. You can also search by subject area to narrow down results further. For example, by call number.