The exact databases you'll use for your topic will of course vary by topic, but here are those most likely to be useful:
Compilation of the most important historical documents and legislation related to immigration in the United States as well as current hearings, debates and recent developments in immigration law. This first comprehensive database includes Board of Immigration Appeals, Precedent Decisions, legislative histories, law and policy titles, extradition titles, scholarly articles, an extensive bibliography, and other related works.
Articles in important academic journals from all disciplines (latest few years not available); also eBooks from leading academic publishers, as well as some images.
Current humanities and social science journals from a variety of university presses.