On a 2006 Chevy Impala 3.5L V6, the Throttle Position Sensor (TPS) is located on the throttle body. The throttle body itself is situated on the intake manifold, near the engine's front. It's a fairly small sensor, usually mounted on the side of the throttle body. You'll likely need to look closely and possibly use a flashlight to spot it. It will have a connector plugged into it.
To be absolutely certain, consult a repair manual specific to your 2006 Chevy Impala 3.5L V6. A picture in the manual will confirm its exact location. Many online sources offer these manuals for download or viewing.
