*shrugs*
Hope it's selling thousands of copies and you need a agent's help to let Simon and Schuster take it off your hands?
Sorry, I don't mean to sound flippant, but unfortunately, unless something like that is the case, the ship has sailed on your book. It's published. What can an agent do for you now? All they can do is sell reprint rights. In the future, if you think you "might" want to pursue an agent, do not publish (self or otherwise) the work you're trying to entice the agent with.
Now, there is a difference between "Hey, this publisher is interested, can I get some assistance" and "Hey, this is already available for sale at Amazon." Agents can sometimes be acquired to help out with negotiations -- a friend of mine's agent was willing to represent me on a specific assignment but I would not have been a full-time client for them (it was a media tie-in deal, not your typical situation).
Do you have another book you can query agents with?