The best database to use for identifying dissertations is ProQuest Dissertations and Theses Global.
Dissertations may be searchable in the catalog by author name, or by including "Penn Dissertations" or "Penn Theses", as well as an academic subject. Here is an example of a subject search for dissertations in History.
Starting in December 2015, Penn required dissertations to be publicly available, although authors do have the option to embargo them for several years. These are hosted on the ScholarlyCommons institutional repository.
Penn Libraries keeps a print copy of most doctoral dissertations completed at Penn prior to 2020, with call numbers constructed from the awarding program's Library of Congress class and a class number that puts dissertations at the very beginning of the numbering sequence (for example, Social Work dissertations can be found at HV001). Within that call number, individual dissertations are arranged by year of award and then alphabetically by author's last name.
We do not collect masters theses, though academic departments sometimes keep them.
The Center for Research Libraries has a large set of international dissertations that can be ordered via Interlibrary Loan.
If you are affiliated with another library or university, you can often request dissertations through the interlibrary loan department at your local institution. Copies of Penn dissertations are also for sale from ProQuest.