Your Mercedes Benz dealer parts department can order replacement keys for you. You will need to bring in your title for the car clearly showing the serial number of your car along with your drivers license, they will make a copy of them and place the order for you. I bought a new master and valet key earlier this year from my local dealer. Even though I have been doing business with them for over 20 years they still require the documentation as that is the Mercedes rule.
Keys made by others will not be the same as the Mercedes key which is made from plated steel, the off brand keys will be plated brass and are subject to wear over time.
While your there ask them to also order a key holder (it is called a "bag" in the Mercedes system, must be some translation issue) for you part number 108-890-06-61 price $16.00 available only in black. With my discount the master key was $25.53 and the valet (secondary) key was $19.89.
The valet key only works in the door locks and ignition, not in the glove box lock, truck lock, or locking gas cap.
Regards,
280SE Guy