Note: Your own local public library is your best resource for local materials! Some local libraries have local history sections with primary source documents, photograph collections, old newspaper collections, etc. They can also advise you on the subscription databases to which you and your students have access. The libraries listed below have digital collections that are national or international in scope.
Note: This is not intended to be comprehensive. Please let us know if you have a research library to recommend!
The Library of Congress
The Digital Public Library of America (this pulls from many libraries and archives, offering primary source documents, texts, photos & more)
The New York City Public Library
The Morgan Library, New York
The British Library, London, England
The Newberry Library, Chicago
The Library Company of Philadelphia