Not certain what exactly the "valance" you're refering to actually is.

If it's the piece of sheet metal just below the black plastic louver grills which are below the front bumper, there is a seperate part that is bolted between the two sections of the front fenders which curl around the front of the vehicle.

#A123 880 00 40 Panelling, under front bumper, central

if it's more than just the central piece there once was a repair panel available for the lower front section of the fenders

#A123 881 03 01 Left
#A123 881 04 01 Right

These parts are listed in the EPC but I don't know if they are still available through the MB dealer