A dissertation or thesis is a document submitted in support of candidature for a degree or professional qualification presenting the author's research and findings. (International Standard ISO 7144: Documentation — Presentation of theses and similar documents).
For most universities in the U.S., dissertation is the term for the required submission for the PhD, and thesis refers only to the master's degree requirement.
The best source to find theses is ProQuest Dissertations & Thesis Global. Policies regarding theses and dissertation collections largely vary between universities. So check the library website of the university of interest.