Often a good topic is both timely and timeless. It has been talked about before and is still talked about now, i.e. an ongoing issue. The tools on this page fit that criteria.
Pick something that interests you. Your speech will be much better (and the research more fun) if your audience can tell you believe in what you are talking about.
CQ Researcher browse by topic @ACU or date @ACU
Covers popular and key issues in the news with a focus on long-lasting policy topics.
Issues and Controveries Topic List @APL
Information on hotly debated topics.