Using databases and indexes is an excellent method for finding resources in your topic. The reason for this is because many resources are compiled together in a digital space so that you can search for pertinent information in a variety of locations with a few strokes of the keyboard. Remember to be flexible with your searches: if you can't find something right away, go back and change your search terms. Ask your subject librarian for help with search terms if you get stuck.
[The main difference between a database and an index is that a database compiles collections of electronic resources (e-books, articles, etc.) so that they are searchable in one interface; an index is a database of citations, often with information about the sources included).
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.
No matter what your topic is the following databases should be searched.