As the previous couple of posts prove, finding a rep is like finding a partner. It's finding a partner for business and creativity. Not all photographers have reps and not all photographers need reps.
There are plenty of solo photographers that I have and will continue to work with. I will be honest though, when starting research for a project the first places I end up going (out of habit) are to the agent sites.
In my opinion you want a rep who is going to help market you and your talents to the right clients for the right project. You'll want to collaborate as much with them as you do with any art director, creative, photo editor, etc.
If you are in search of an agent just remember it's as much of an interview for them as it is for you.
I won't be posting the next few days (off to a weekend wedding) but I am compiling a big list of agents and will post all the links here next week.