Where you should search depends upon your topic.
Overall, a good place to start would be the Bibliography of Slavery and World Slaving.
Searching for journal articles on slavery in South America? Try Historical Abstracts with Full Text.
Looking for general information on slavery? A multi-disciplinary source would be best: Academic Search Ultimate.
Searching for journal articles on slavery in the U.S. or Canada? America: History and Life with Full Text.
Additional sources for finding newspaper, magazine, journal articles are listed to the right.
Image Source: Advertisement for fugitive slave. Maryland, 19th century. Chicago Historical Society NW0305, as shown on slaveryimages.org, compiled by Jeromy Handler and Michael Tuite and sponsored by the Virginia Foundation for the Humanities and the University of Virginia Library.
350,000 fully searchable images of pages from over 700 historical American newspapers. Focus is on the 18th and early 19th century.