BMW Repair: Question re service engine code, 2000 bmw 328ci, bmw 328ci

Question
Hi Evan, car is a 2000 BMW 328ci, last 7 of the VIN is JN91327.  My check engine light has been coming on and going off for some time and when I went to my repair man he told me that the code was a P1250 and that I needed to replace the oxygen sensor and something about a bank 1 senor 1.  I had an emissions test done today because the service engine light has been off for about 3 days now and I failed, due to "on-board computer is not ready to determine the status of the pollution control systems on the vechicle."  The inspector advised me to drive around for a week or 2 to complete the drive cycle.  My question is what type or sensor do I actually need to purchase?  The repair man told me it would cost about $250 for the sensor but he tried 3 places and had no luck finding the sensor.  Should I be asking for a bank 1 sensor 1 sensor when I call around to different BMW service parts centers or just an oxygen sensor. I'm obviously more than a little confused.  Also, do you think that before I purchase anything I should just drive around for a week or 2 to complete the drive cycle (b/c maybe it'll pass this time around)and exactly how many miles does it take to complete the drive cycle for my car?  Any help you can give would be most graciously appreciated.  Thanks.

Answer
Holly, the O2 sensor part number is 11 78 1 742 050
couldn't get the price on it. If your vehicle has over 100K miles then I would suggest replacing both Pre-catalytic converter Oxygen sensors. You have 4 Oxygen sensors total. The pre-cat sensors are considered sensor 1's and the post-cat sensors are considered sensor 2's
Bank 1 is Cylinders 1-3 and Bank 2 is Cylinders 4-6. Holly I would also pay for a DME software update to avoid the possibility of this being a software issue. There has been issues with faulty engine computer (DME) software causing check engine lights with O2 sensor faults. Should cost an hour's labor.
-Evan