I am sure some "guy" somewhere can reprogram a used KeylessGo card to your car, but I would not count on it. If you can still get a dealer to order one for you, that will be the easiest and least expensive router. Like I said, I am not sure if they are still available. For some reason, my feeble memory thinks I might have seen or heard where they had been discontinued, but I could certainly be wrong about that - your dealer will know for sure.
Bobterry's suggestion of replacing the car's KeylessGo module with a newer version so you can use a new SmartyKey instead of the card would be the only real solution if you can't get a new card. It is a bit of a complicated task, though. You will have to locate a proper KeylessGo module, install it, code it with SDS (Star Diagnosis, the official MBZ shop tool) then have the dealer change their records to indicate that your car now has the updated KeylessGo system and have them order the proper SmartKey with that feature built-in. It's not going to be easy or cheap