These are available on Ebay for $35-$45 for a pair. I bought them from Turkey, but ended up not needing them. I found the same part at a salvage yard, which is why I don't need this anymore. Anyway, if anyone wants them for $18 shipping included, I am happy to send them. In looking at the quality, I would say they are fine but not amazing. Unfortunately, MB doesn't sell just the mirror gasket, so other than a salvage yard, this is the only option I could find to improve that part of my car.
You will have to cut holes in the middle of the gasket to fit your mirror wiring through it. I sealed the holes afterwards with plastidip to improve the water protection. Shipping in a padded envelope or small box is ~$8, so I am netting $10 here. Thanks.