Seal is readily available from Honda - part number 91208-KB4-671. If you have a Honda powersports dealer near you, I'd see if they can get this without having to pay more for shipping than the seal itself.
Replacement is pretty straightforward - wood screw carefully through the center of the seal ring, pry it out, clean the seal area and press the new seal in (a length of 3/4" or 1" PVC works really well here). I prefer a super light coat of hondabond on the outer edge of the seal surface on this one, since it seems to love to fail regardless of the bike model.