So recently I replaced my entire exhaust system from stock, to a 4-2-1 header with 2 and 1/4 exhaust all the way back. The exhaust I chose is catalytic converter eliminated. So naturally I had to get O2 sensor spacers to compensate. The manifold and exhaust system I got was cheap aftermarket off eBay because I needed something to pass New York state inspection (I know typical). So the manifold didn't properly hold the stock position of the O2 sensor, so I had purchased an angled O2 sensor spacer. So I installed that in the manifold with the O2 sensor. And then after the catalytic converter I installed another spacer with the O2 sensor. After doing so I noticed the vehicle had a rough idle either at lights, or in park. Is it possible that the O2 sensors are throwing off my ECU? Or are the O2 sensors not sensing the proper amount of gases through my exhaust due to the spacers?

 

You can use the 'spacer' on the after cat O2S to keep it from chucking a code, but on the front this will diminish its ability to read the exhaust gases and will not be able to properly compensate and adjust for changes in the A/F.

Not sure what you did for the primary O2S. On the B2 the O2S is scewed into the bottom of the manifold to read all the cylinders, some cheap aftermarket headers only screw into one of the primary tubes which makes it very accurate for that cylinder and inaccurate for the rest.
Extend the O2S harness so that the O2S sensor is in the collector of the pipes to read all cylinder gases.
On EX models the primary O2S is located right in front of the catalyst, located below the oil filter. If you have changed your exhaust from the factory 4>1 to the EX style 4>2>1 then the collector will be under the car and that is where the O2S needs to be.

 

Would a ~30 dollar Denso oxygen sensor found on the internet be a good buy to replace an old stock one in 4th generation Honda Accords in terms of preventive maintenance?

 

Thats what replaced mine with on my 97..works so far..