a/f sensors
 

I am looking for a pre-catalytic converter rear manifold a/f sensor for my 99 ES300. However, in all the searches i do online, I can only come up with simple O2 sensors. I'm not sure that I can just substitute an O2 sensor for an a/f sensor on a Cali emissions vehicle. Can anyone provide any expertise on my options or where I can find a 99 ES300 a/f sensor?