Just bought four Bosch 4-wire O2 sensors from
www.oxygensensors.com. They were $112.00 each, and had the proper connectors (no soldering, crimping, etc.!)
I've discovered that one can probably use a universal 4 wire sensor as John is suggesting and will try out for us soon, but you had better be willing to match wires and recycle the connectors off the old sensors... this concerns me in that they're exposed to heat (they age poorly) and I'd probably get a wire wrong and screw it up one way or another.
Trivia you may not know:
Our oxygen sensors have 4 wires... two for the going and coming of the information required to sense the exhaust, and two to power a heater to the unit so it can begin working efficiently, even when the engine is not up to temp.
There is also an 02 sensor heater relay that my mechanic is suggesting I buy and replace because that too came up on the PSTII tool as being faulty.