Use this site search key and look for 'heater matrix'. The company making replacement matrix also do radiators. However if you use your local yellow pages you should find a local motor-rad that repairs or re-cores your radiator. If its the usual Sportrak leak I expect your radiator has rotted along the top seam or beside the top hose.
I managed to repair my radiator recently - stick of soft solder and a flame/torch. Most difficult task was releasing the lower hose to remove the radiator from the car.