How do I look for primary source material in the history of medicine?

The printed Index-Catalogue of the Library of the Surgeon-General's Office (online version IndexCat), U.S. Army (61 volumes in 5 series, 1880-1961) is a valuable resource, especially for pamphlets, dissertations, and journal articles from the 19th and early 20th centuries.
Index Medicus (1879- ), which indexes articles in the medical journal literature, is another useful resource that is widely available.
The Bibliography of the History of Medicine, a printed version of the discontinued HISTLINE database, was published in six cumulative volumes (1964-1993), and is useful for pre-1993 material.
For more information, see Printed Catalogs and Guides.