Mac - Publisher's Lunch is free. You can subscribe to their daily email.
I don't know of any site (free or paying) that allows you to search for authors and then see who represents them.
What I did when compiling my list - was I looked at books similar to mine and checked the acknowledgements - authors often thank their agents by name.
For those agencies that have websites - almost all list their clients - although they don't always designate which agent represents which client. You can always call the agency to find out which agents represents a specific client. Also many authors have their own websites and their contact (agent information) is almost always on their sites.
You should also go to agentquery.com - you can search agents by the genres they rep and then you can click on the individual agents that come up to check their client list, recent sales. etc.
There's no one-stop shop - although there are some directory's you can buy like Jeff Herman's annual guide.
It takes a lot of time to do the legwork to compile your list of agents. Always check preditors and editors or writers beware to make sure an agent is legit.