I have looked at this a little bit. And yes, it has been suggested by lpcapital here

If you need the part, BMW of Bridgewater still sells older releases of the sensors for $220.
This part number works for both rear and front sensor (they are same essentially, one with slightly longer cable):13628511666

Upstream NOx sensor in my vehicle threw fault at around 90K miles, I replaced it. Now, downstream is throwing the same code. Just ordered the above sensor while in stock. Hopefully good for another 50K miles form here on.

These sensors are essentially a maintenance item, expected to last about 50K miles. Seems to be so in my case. Both replaced at 49K under warranty. Upstream failed at 90K, downstream failed at 108K. Go figure.