You have two choices.
(1) you can buy the complete gland (the seal and the screw-in part) from Merc dealers. You need two.
(2) you can remove one, take it to a good hydraulic shop and they can match the seals perfectly. You can “pick” ’em out of the gland, insert new ones, and be as good as new. I just did this. The two trim cylinders are easy.
For either option, as all you need is a spanner wrench to break them loose. I bought one on Ebay that has a half-dozen different sized pins to provide a tight fit, as well as an adjustable spanner so that you can also remove the center tilt seal if you need to. That one is a bit harder, but I can talk you through it if you want to replace it too, I just replaced all of mine maybe 3 months ago, for the same reason you gave, that “sheen.”