First off, the options are somewhat limited. In a 2003 interview with Gamasutra, Mark Long states: "shopping for an agency is a pretty simple task right now, since only a handful exist". Unlike other fields, game development is comparable new. Also, there's the theory offered in a NY Time's piece on game agencies that "a deep cultural divide between the slick, Armani-suit-wearing agent crowd and rumpled computer-code-writing gamers has proven difficult to bridge." Both these pieces are a bit dated, but there doesn't seem to have been a huge amount of growth since then.
That being said, there are some agents & agencies out there. Starting with a web search for "game development agents" I was able to find one agency straight off. It turned up a couple of articles (such as those linked above) that in turn lead me to a few more. In short, you're going to need to grind the web & do some professional networking to work up some leads.
That being said, I also want to address what seems to be a bit of a disconnect in the question comments. You state that you're not looking for any money or funding because your app is already finished. That may be true, but marketing your game to a publisher still represents an expense to somebody. Your "future profit sharing" proposition requires the agency to take a calculated risk their expenses for finding you a deal will be offset by future revenue. There's often a considerable financial gap between developing a game & its profitable release - even previously successful AAA studios struggle with this.