Where do you find the tire sensors?

Answer 1

Inside the rim. These are pressure transducers that send a radio signal to the ECM/BCM (car's computer) and tell the car what the pressure is inside the wheel. The signal is then read on the console of the car and more sophisticated vehicles present a value like 30 psi, less sophisticated vehicles only alert when there's a reading that's "out of range." If they're bad or sending incorrect signals they can often be reset by the user interface on the car's console. If the transducer is bad, it will have to be replaced and a "handshake" sequence initiated for the car to recognize the new transducer.

This is advanced technology, but is more common place on late model vehicles.