sc300jon

On my '92 SC300 there appear to be at least two O2 sensors: one near the front of the exaust manifold and another towards the rear. But I've also heard there is yet another hiding downstream near the catalytic converter.

That, and these sensors tend to be very expensive; I've gotten quotes ranging between $90-100/pc!

Can anyone confirm the location/qty of O2 sensors?
Anyone know of an affordable source?

Thanks,
-Jon