You can buy souvenirs at the Dairy in Central Park. It once was intended as a source of fresh milk for children, but now it is used as an information center and as a gift shop. Most of the building itself, located at 65th Street and it is open Tuesday through Sunday, from 10AM to 5PM. http://www.centralpark.com/guide/attractions/dairy.html
Belvedere Castle (mid-Park at 79th Street) also offers maps and souvenirs of the park, as well as a kiosk during the warmer months at Columbus Circle. http://www.centralpark.com/guide/attractions/belvedere-castle.html