Journal articles provide in depth scholarly information. They are vetted and improved by peer review. They are usually fairly short in length and focused on discussing one specific issue. The following indexes are good places to find journal articles about history.
These indexes are particularly good for accessing the scholarly literature of specific disciplines, i.e., articles written by historians, Asian studies scholars, and literary scholars.
Usually researchers find more sources by looking at the footnotes in an article or book, but these will always be older than the publication you have in hand.
Citation indexes like the Web of Science (which includes sections for the Social Sciences, Arts and Humanities, and Science) are set up to search for sources cited in the footnotes of journal articles as soon as they become available.
This allows you to find newer articles which cite the books and articles you already know are key for your topic. By relying on connections between authors rather than subject words and by moving forward in time, citation searching can open up new avenues of research.