Where you should search depends upon your topic. Library databases that will be helpful may be found below.
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 Ultimate.
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.
Provides citations to articles on the history of the United States and Canada from prehistory to the present. The database covers over 2,000 journals published worldwide.
A bibliographic database that indexes world history scholarly literature from the 15th century to the present. Provides citations to journal articles, magazines, books, and dissertations from over 2,300 journals. Covers all countries and regions except the United States and Canada, which are covered in America: History and Life.