The throttle position sensor is built into the throttle housing and isn't changeable, so I've read, so would need to buy a complete throttle.
From what I've read the throttle position sensor is built into the accelerator pedal assembly. The butterfly valve in the throttle housing is driven by a servo motor to follow the pedal position. In other words it's a drive-by-wire system.
On the 1.8 engine the throttle housing is at the front of the engine.