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.
Center for Research Libraries: Foreign Doctoral Dissertations
CRL has more than 800,000 cataloged foreign doctoral dissertations from more than 90 countries and over 1200 institutions.
Carnegie Mellon theses are now ONLINE and can be searched through the ProQuest database Dissertations & Theses @ Carnegie Mellon University that enables access to citations and abstracts of all dissertations and theses, as well as the full text in PDF format. Scroll down and select Dissertations & Theses, then do a regular search. Print versions are also available in the libraries' collection.
PRIMO, the Carnegie Mellon Library catalog, uses the term THESIS to denote both masters' theses and dissertations. However, the number of master's theses is limited. Within the libraries, theses are located in designated areas and are shelved in alphabetical order by the author's last name. The catalog treats theses and dissertations like books, and they can be borrowed as such. Theses may be in print, microfiche, or microform.