If the leak is coming through the spotlight the gasket would be part of the retainer plate on the outside. The gasket and plate are one piece and cannot be sold separately. 3 hole retainer plate needs to be used.
It is also common that the leak may be from somewhere further up in the post. When the water hits the shaft of the light while traveling down through the inside of the post, it moves into the vehicle on the shaft making the leak appear to be from the spotlight.
Left side plate:
http://www.unityusa.com/Retainer-Plate-Kit-Left-3-holes_p_2136.html
Right side plate:
http://www.unityusa.com/Retainer-Plate-Kit-Right-3-holes_p_2137.html