You need to find a distributor. A distributor is essentially a middleman company that sells your book into major accounts such as Barnes & Noble. Getting a distributor can be just as difficult as securing a publisher—you have to convince them there’s an audience for your book and that it’s a quality work. A couple of the well-known distributors are Publishers Group West and Consortium. Distributors will ask you for a substantial discount (as much as 60 percent off retail price), then they will sell at discount to their accounts (usually at 50–60 percent off).
If you don’t have a distributor, you are not likely to get your book into stores nationwide, though you may have some luck getting your local or regional bookstores to stock a few copies.