Controversial issues are topics that are divisive. Many of these topics have been divisive for a long time, such as abortion or capital punishment. Many people feel very strongly for or against these kinds of issues. For this reason, many of the sources you find on these topics will be biased--they will be persuasive articles or books that are trying to make you think a certain way about the topic. Just so you know, there are also objective, academic articles that are meant to inform rather than persuade the reader.
You can still get factual information from some persuasive articles, but you have to be more careful so as not to produce an unintentionally biased paper or presentation that is meant to be objective. Most importantly, you have to be able to recognize if your source is biased and if it is appropriate for your assignment. If you aren't sure, ask your instructor.
CQ Researcher, Issues & Controversies, and Opposing Viewpoints or Global Issues in Context are the top databases that specialize in controversial topics. They provide pros & cons or viewpoints that are persuasive as well as objective information.
CQ Researcher is well known for being objective and thoroughly researched by their professional writing staff. Each full report on a topic has a table of contents with different sections that have specific kinds of information. If your topic is covered in CQ Researcher, you'll have a great source to get you started.
The In Context databases are a little different than CQ Research and Issues & Controversies in that they provide viewpoint articles by their professional writing staff in the Opposing Viewpoints Online Collection, but they also have academic journal articles, news and magazine articles, media and more on the topics or issues.
The top academic databases for controversial issues are are Academic OneFile, Academic Search Complete and Research Library. In these databases you'll need to search to see if your topics is covered. They are the best to try if you need scholarly and peer reviewed sources.