NeedsWings Performance Products. Ignition XF
U.S. key was $159.95, shipping extra, no tax for me. I doubt you will find a new OEM one perfectly cut/programmed to your car for less money (at least that's also the best I spotted after people here recommended it.)
My used Crossfire I just bought in February also only had one key.
Funny thing, the bag it was in, was from a Mercedes facility, 15050 Frye Rd, Fort Worth, TX, about an hour from my house. They shipped it to Michigan to NeedsWings, who shipped it back to me !
Here's the programming instructions that came with it. As it comes to you, it will start the car, but you need to program it to lock and unlock the doors.
1. Insert the key into the ignition switch.
2. Press either the lock or unlock button momentarily twice. Turn the key in the ignition switch to the ON/RUN position within 30 seconds.
That was it, and it works fine.