Just Failed CA Smog. Now what?

[Okiedoke]

I bought my Z just over a year ago. Presumably it would have had to pass smog at that time. This time, everything passed except for NOX. Max for the 15MPH test is 545 PPM. My Z measured 587 PPM. The 25MPH test passed with 459 PPM. The Max is 502 PPM. The car is stock except for a KnN filter. I just went through some of the CA smog related threads and there seem to be folks who got a measurement in the 200s. In this thread by Z51JEFF, it looks like he passed by replacing his O2 sensors. Is this what I should try? If so, does anyone know what model and where I can get them and how many? Is there any way to tell after replacing the sensors whether it will work short of taking another smog test? Thanks!