I just had a look at the tail lens gasket on my SP project- it's looking a little tattered too. If you can't find the gasket:
Buy some (thicker) engine gasket material, trace the pattern using the tail light lens. With a pair of scissors and a set of leather punches (Cheap at Harbor Freight) to make the screw and dimple holes and to cut inside corners, it's pretty easy to make one. Probably quicker than ordering one too.