Letters & diaries published as books can be found in the UM LIBRARIES CATALOG by adding keywords:
See also: Missouri Documents guide.
Historians will point out that newspapers are not really primary sources since they are not accounts by the historical actors involved. However, they do constitute at least contemporary writing/speaking about the incident that contain good information about the impression available to contemporaries about an event.
For further information about federal government sources, see the guides to Federal Government Documents and the Primary Sources By Decade page. For Britain and foreign governments that were part of the British Empire, see the Parliamentary Papers guide.
For many more polling resources, see Sandy Schiefer's extensive guide on public opinion polls.