The industry bible is Red Books. They are online now at
http://www.redbooks.com and have every major agency with billings over $250K in their database.
Its an expensive site to subscribe to, (I bought the $1,600 annual subscrition which is nutz), but worth every penny if you nail some good agency deals out of it.
Be real careful Brisk. These PR and Ad agencies can take you for a ride. I had one last year that we almost signed for Men bought they wanted $8,000 a month to run the PR campaign which at first looked good in the initial presentation, but when we looked more into it, it was a lot of hype and there would have been more costs that weren't discussed after we signed them.
Most PR agencies work on a retainer based on the size of your campaign. Ad agencies make their money from either a 15% to 20% commission or the markup if they are buying spots wholesale from you.