PSU Library has a large collection of reprinted or microfilmed primary source materials, including the collected writings of historical figures, government documents, diaries, collections of speeches, etc. The best way to find primary sources in the library catalog is with subject headings. You may have to try several searches, but one strategy is to combine a keyword for your topic with a subject heading for a material type (see list at right).
I did the search above, and this book was the top result. I looked at this book's record to find some more subject headings I might want to use.
An excellent library database for primary sources relevant to the Pacific Northwest.