A dissertation is a thesis submitted as a requirement for a doctoral degree. Library catalogs often use the technical term thesis: e.g. "Thesis (Ph.D.)." There are different types of doctoral programs in music. The most frequently encountered are
D.M.A. (Doctor of Musical Arts): a performance degree
Ed. D. (Doctor of Education)
Ph.D. (Doctor of Philosophy): musicology, ethnomusicology, theory, composition.
While D.M.A. dissertations may be particularly useful to performers, Ph.D. dissertations tend to examine the topic in greater depth and to offer more extensive bibliographies.
You can find dissertations and theses in the following databases or order via Interlibrary Loan:
Official digital archive of the Library of Congress and the database of record for graduate research. Search citations to dissertations and theses from around the world from 1861 to present day, and access full text dissertations.