To find book primary sources, use library catalogs like Barton and MIT's WorldCat.
With a BPL card or e-card, you can access a wealth of online historical primary sources.
It's not cheating to check the footnotes and bibliographies in your secondary and reference sources (books, journal articles, and encyclopedias) to identify primary sources for your own work. Following footnotes is an approach used by many scholars to identify not only relevant seconary literature, but also primary source bases.