Follow this link to a thorough guide - with good links - to intelligent searching for and use of primary sources. It was produced by the Reference and User Services section of the American Library Association.
A few tips:
- Use the same "marker" words you would while using the catalog (e.g., "diaries," "letters OR correspondence")
- Limit your search to sites affiliated with educational institutions, nonprofit organizations, and governmental organizations
- add "site:.edu (or, .org, .gov, etc.)" to your Google searches to restrict the domain
- use a clustering search engine like Yippy, and examine the cluster by domain
- Try looking for local historical societies close to your subject's place (e.g. "johnson county historical society" AND "civil war")
There are many good collections of primary source material available online. Try some of those listed below, but remember that there are always more for you to find.
Try this instead of plain old Google to do a focused search in selected educational and non-profit domains.