PhD dissertations sometimes include a literature review on their topic. These reviews can be helpful in building your bibliography.
Once you have generated a list of books and articles, you can further build your bibliography by investigating where those books and articles have been cited in other books and articles.
Reference works can provide a broad overview of a topic, and also furnish a starting bibliography. We carry encyclopedias, chronologies, necrologies, and other standard reference sources in the history of medicine, including: