There are lots of ways to find agents. In my case, stand on a street corner near Penn Station, pop open a bottle of Macallan 12 and you won't need to wait long.
In the case of other more sober and circumspect of my ilk, there are resources like AgentQuery.com, AbsoluteWrite.com and the annual published editions of Guide to Literary Agents and a myriad of other, lesser known sites as well.
Not all of them are created equal.
If you use any of those sites, check to see if there's a way agents can update their info, or give feedback to the site manager.
I've found a lot of sites that purport to list essential info about agents that have my details including sales and what I'm looking for, just plain wrong. I'd send them an email to fix it, but there's no way to do so.
And when I get a query that references one of those sites, I know I'm looking at a query from someone who didn't do much research.
Research can be over-rated. There's no way to make a list that won't have a bad fit or two or ten on it. The trick is to realize research means checking more than one site, starting with the ones that are most up to date, and most readily correctable.
My list of where to start: AgentQuery.com; QueryTracker.net, AbsoluteWrite.com, PublishersMarketplace.com
If you've got others, list them in the comment column, and I'll be glad to check them out.