Chances are, you'll need to find an established company to manufacture your product--either for a fee or in a joint venture relationship. For simple products with a one- or two-step manufacturing process, you can find manufacturers in a state manufacturing directory. Each state has such a directory, available in larger libraries, where you'll also find the Thomas Register of American Manufacturers (http://www.thomasregister.com), a directory of manufacturers nationwide.
Another way to find manufacturers for simple products is to locate distributors of the equipment used to make your product; they can refer you to manufacturers they sell to.
It's harder to find manufacturers for products that involve many different manufacturing processes. Some manufacturers advertise in trade magazines; Gale's Directory of Publications and Broadcast Media (at your local library) lists most trade magazines.
You can also contact distributors of parts you need and ask them what manufacturers might work with an inventor. If distributors in your town can't help, call ones in large nearby cities.