Unfortunately, very few publishers of any size take on unagented authors. There are some out there, but you will have to accept that they will be unable to market and distribute your book like a larger publisher would. There are exceptions, of course. Check your current Writer's Market.
Better than Writer's Market, Writer's Digest Books also puts out a "Guide to Literary Agents" every year. It should tell you what you need to know. An agent is the best move to make in the publishing world. They know the business, will represent your interests, and without one, it's very difficult to get a foot in the door.
In a long shot: you might also find out who your favorite authors' agents are to get a feel for who represents the type of work you'd like to publish.
Getting an agent is difficult. It's like having to get published before you can get published. But it can be very worth it. Good luck!