Craft stores will carry paint markers. The markers are usually located in the writing or drawing aisle. Examples of craft stores are A.C. Moore, JoAnn Fabrics, Hobby Lobby or Michaels. If you cannot locate them, ask a sales associate to guide you.
Large discount department stores, such as Walmart or Kmart carry paint markers. They will be located in the arts-and-crafts section with other drawing and painting tools.
Paint markers are available to order online from many retailers including eBay. An online retailer that carries paint markers is http://markingpendepot.com/.