Bison, most commonly known as buffalo, are the new American national mammal and there is no better place to see and photograph them than South Dakota.
Custer State Park, Wind Cave National Park, Badlands National Park and Bear Butte State Park are places you are almost guaranteed to have a close encounter of the bison kind. They are also found in private herds at various spots around the state, including on several Indian reservations.
Photographing buffalo in the parks can sometimes be as easy as driving up and shooting out the window. If you do venture out of your vehicle around bison, be extra careful. These are huge beasts and very quick in reacting to situations they don't like. Approaching a bison is a really bad idea, trust me.