Maybe check this out may be the right size but you would have to buy the whole kit.
Or depend on the style and how its made you might be able to replace the seal. I've never pulled one from an S type so not sure how the seats and seals are set up. Would you happen to have a pic. If its a standard O-ring seal you can buy silicon o-rings if you can measure it or there are A/C o-ring kits that you could try.