Are you talking about the input shaft seal, or the case gasket?
If its the input shaft seal (the seal on outside of supercharger, on the pulley side) Napa sells the seal, part # 11625 I just had to replace this one on my car....its the identical seal, all the way down to the numbers stamped on the outside of it. The part is about $6.50 Napa calls it a Rear pininion seal, but dont get discouraged its the correct seal.
If your talking about the case gasket, just run a small bead of silcone or rtv around the channel on one side...make sure you let it dry before you re-install the supercharger.