It's best to teach a man to fish...
If you don't know about it already, there is a website called realoem.com, which has the parts diagrams for BMWs (it's called the BMW TIS). You can use this site to find part numbers for whatever part you need. All you need to do is plug in the last 7 digits of your VIN and the TIS will bring up the production information for your car and all the parts in it grouped into several different categories.
The 320i is not sold in the USA so I'm not familiar with it, however cars are cars, so this is the parts schematic for the N62 4-banger, which is probably the engine in your car. Bank 1 Sensor 1 is No. 2 in the drawing. It looks like the N62 has two cats built into the mainfold and has four O2 sensors - two for each catalytic converter.
Good luck with it.