You can go to any of the big bookstores . . . Don't try to sell your first book until you've finished it. Unpublished authors often have great ideas that peter out after a dozen pages. You must prove to prospective publishers that you can sustain writing quality, and the only way to do that is to present them with a completed manuscript. Get a literary agent.
Publishers routinely ignore unsolicited manuscripts.
They use agents as screeners. Yes, they will take anywhere from 10 to 20 percent of the money you make, but 80 to 90 percent of something is better than 100 percent of nothing. It is often harder for a writer to find an agent than for an agent to sell a writer's book to a publisher. These books should be readily available in any area bookstore. Make a short list of agents who seem most interested in representing the kind of book you have written.