Turn to these free online primary sources:
Medical Heritage Library
A consortium of medical libraries, including Yale, have put parts of their collections available online.  Over 300,000 freely downloadable primary source books, many from 1780-1923.  On the Medical Heritage Library website, under the Content tab, you can find primary resource sets on vaccination and disability, as well as access American and State medical society journals.

National Library of Medicine (NLM) Historical Collections
Great primary history of medicine material, including digitized books, archives, and moving images (aka movies). Also see their huge image archive, which includes posters and more!

Internet Archive
Home to a huge variety of resources, including Moving Pictures, for free use.  Not limited to history of medicine, but lots of possible primary sources here.  Make sure, if you pick a source from here, that it's uploaded by a library or other institution!

Hathi Trust
A huge online repository of freely downloadable books and journals, from a collaboration of libraries including Yale.  Not limited to history of medicine, but lots of possible primary sources here.

Digital Public Library of America
This site pulls in sources and archives (like letters and diaries) from many libraries, museums, and other institutions, as well as big databases like Hathi Trust and Internet Archive.