I think the dealership can order it for you with a matched key if you go to the dealer and use the vin number on the car. You might have to fill out some heavy duty paperwork and provide proof of ownership. Check into it, it may help.
Otherwise have the ground wires checked and the wires on the starter to see if they are loose, but everyone else seems to think its the switch, which may just be your best bet.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.