If time is not critical, order a new tumbler to match your VIN (~$100). It will come with a new key you can try. If it works, DON'T 'UNTWIST' THE KEY UNTIL YOU CHANGE THE TUMBLER. The worst case is still having the dealer take care of it. You just bought the parts in advance.

You can call a locksmith, but I'm sure he'll quote from the Mercedes owner column of the price list since your car has 'special' locks and keys.

Unless your tumbler stuck for the first time today, you waited too long to replace the tumbler.

Sixto
91 300SE
87 300SDL
81 300SD