WorldCat, for books other libraries have and will send here for you via Interlibrary Loan (ILL).
Definitely read the detailed tips on the Find Books page of this guide to most effectively use the catalog, WorldCat, and ILL.
Definitely search both:
America: History & Life or Historical Abstracts (depending on whether your topic deals with the USA/Canada or not) for discovering articles from the greatest number of history journals and for more search tools, including the Historical Period limiter.
JSTOR, for finding articles from those journals for which we have full text available through the JSTOR database.
Definitely do each of these:
Look for the primary sources used and cited by your secondary sources. Mine the bibliographies of relevant books, articles, and dissertations.