as mentioned Sturmey-Archer S3X 3-speed fixie internally geared hub kit rear 36h 130mm Black, Hubs if you want to change gears without stopping. Keep in mind you shouldn't back pedal with this hub and should only be stopping with brakes, that means no skidding.

Otherwise a double fixed rear hub with a quick release would be your best bet. You will have to take the wheel off and flip it to change gear ratio's.