Originally Posted by M60A3Driver

I think it comes with the seal in it. Here's an example of one: Expansion tank cap
If you need it fast you can go to a Mercedes dealer (hopefully they will have it in stock). You will need a VIN from an SLK 320. Find one for sale and use the VIN from it.
Bring your original cap with you. They might even be available at a regular auto parts stores.