I had an O2 sensor installed in my 89 535im about 6 months ago. It was AU$230 ish for the part and $50 to fit.
The part I got was a Bosch part the same as the dealer part except that there is not plug on the end of the wiring, so it had to be joined to the old O2 sensor wiring.
I had a local european specialist auto electrician do the job and supply the part.
Myles (Brisbane)