• In addition to materials in the Sophia Smith Collection (sse link at bottom), there is an abundance of primary source material in Neilson Library.
• Use secondary literature for leads on primary sources.
• As you search the online catalog; watch for the words "personal narratives," "interviews," "correspondence," "diaries," "pamphlets," and "sources" as part of the subject headings. Other clues may be buried in the title, authorship, date of publication, etc.

Try Keyword searches such as these:  

• (diaries or correspondence or narratives or interviews or sources) and 1914-1918 and women
• (diaries or correspondence or narratives or interviews or sources) and (war work) and france
• canteen* and 1914-1918 [for something this specific, don't bother to add diaries, correspondence, etc.]
• Note that there's no single "right" way to do this and that you'll need to play around with various combinations.

Or use Subject Begins With for these:

• World War, 1914-1918 - Personal narratives
• World War, 1914-1918 - War work - France - Red Cross - Sources
• World War, 1914-1918 - Women - France - Personal narratives, American