There are two exhaust manifolds. One of the two pipes running from them to the catalytic converter has the sensor. It's just ahead of the cat. The wires to the sensor feed up to a connector that is under the curb side carpet in the footwell for the front seat.
Remember to use anti-seize paste when installing the new sensor.
The Bosch sensor lists for $170 but is sold here (Mercedesshop) for $128.