Try KEYWORDS ANYWHERE to find a few relevant titles. Then track through on the Subject(s) listed on individual book descriptions to conduct a more comprehensive subject search.
As you look through lists of subjects, watch for the words "personal narratives," "diaries," "interviews," "correspondence," "pamphlets," and "sources" as part of the subject headings.
You can also search for these words, for example, gold rush and (diaries or narratives or correspondence)
Other clues may be buried in the title, authorship, date of publication, etc.
You can limit your search to items published 1885-1905 by using the advanced search option. BUT, you'll lose any modern reprints of sources originally published 1885-1905.