A steering wheel knob can be purchased from a number of different places. It is important to check with your local police department regarding the legality of a steering wheel knob. They are illegal in a number of states and cities. You can buy a steering wheel knob at the majority of auto parts stores. Big box retailers such as Wal-Mart and Target often carry steering wheel knobs. They can be found online as well. Amazon also carries a selection of knobs. These knobs are often installed on lawn tractors or in handicapped equipped vehicles.
For more info, see How to Install a Steering Wheel Spinner Knob