National Archives

https://www.archives.gov/research/catalog

Library of Congress Primary Source Sets

http://www.loc.gov/teachers/classroommaterials/primarysourcesets/

Digital Public Library of America

https://dp.la/

Primary Source Sets

https://www.archives.gov/research/catalog

Gilder Lehrman Institute for American History

https://www.gilderlehrman.org/collection

How to Cite Primary Sources in MLA Format

http://www.loc.gov/teachers/usingprimarysources/mla.html