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
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