I cannot recommend AgentQuery enough. This site is how I find nearly all of the literary agents to whom I make my submissions (see what I did there — thank you Nicole I get whom now), and full of very useful information.
I started with AgentQuery but I found a few other sites that I just personally loked better. Nothing wrong with AgentQuery, just wasn't for me. But another really good resource for researching thr actual agents is publishersmarketplace.com. It does cost some money, but it provides you the latest information on book deals, agents and the different books/authors they represent, and it's highly regarded as the place to get most of that info. Several big authors recommended that site to me, and it's totally worth it.