Where do I find an affordable O2 sensor.
 

My AT 04 TSX needs an 02 sensor, want a good brand but can't afford $300+. Any ideas.

Don't go with just any brand though. There are dozens of accounts of brands like ACDelco and Bosch O2 sensors not working correctly for Honda's and Acura's.

Denso is the OE manufacturer for most Honda O2 sensors. Rockauto has both upstream and downstream O2's. You'll see little hearts on the Denso's because they're the most commonly purchased sensors. Because mechanics frequently purchase from Rockauto and most know that Honda's are picky.

Downstream looks to be about $47 and Upstream is about $155:

https://www.rockauto.com/en/catalog/...2)+sensor,5132

Upstream is always more since it's more than just an O2 sensor. It's an air/fuel ratio sensor which is more complex than the simple O2 sensor on the back end of the catalytic converter. You didn't say which one you need but I'd go with one or both of those depending on which one you need to replace.