Doctoral dissertations (and, to a lesser extent, Master's theses) can be very useful resources. The works themselves may not be peer-reviewed in the same sense that scholarly journals are, but they have been reviewed and critiqued by the writer's doctoral committee, who are frequently scholars in the same area of study. Furthermore, dissertations are replete with references -- often even including a thorough literature review -- that can be a veritable gold mine for someone looking for related sources.
William & Mary Libraries' best resource for finding the full text of doctoral dissertations is ProQuest Dissertations and Theses Global.
ProQuest Dissertations & Theses Global is the world's most comprehensive collection of dissertations and theses from around the world, spanning from 1743 to the present day and offering full text for graduate works added since 1997, along with selected full text for works written prior to 1997. It contains a significant amount of new international dissertations and theses both in citations and in full text. More than 70,000 new full-text dissertations and theses are added to the database each year through dissertations publishing partnerships with 700 leading academic institutions worldwide, and collaborative retrospective digitization of dissertations.