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 slavery in South America? Try Historical Abstracts.
Looking for general information on slavery? A multi-disciplinary source would be best: Academic Search Premier.
Searching for slave information in the U.S. or Canada? America: History and Life.
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.