Which aftermarket catalytic converter for 2000 Corolla

I have a 2000 Corolla, 200K miles with DTC P0420.  It has a new spark plugs, few year old MAF sensor, and two new O2 sensors.  The P0420 comes back every time soon after I clear the code with a scan tool. (Actually the code comes back quicker after the O2 sensors are replaced by new ones.) The scan tool cannot make plots, but every time O2 sensor 1 has low voltage (high O2 level), sensor 2's voltage also goes low within a second.  I think this means the catalytic converter cannot remove much O2 from the upstream.  I think I should still check if there are leaks in exhaust systems and fuel injectors.  

If I ended up needing a new catalytic converter, which brand, model would you recommend?  I heard that some aftermarket ones don't work very well.

Thanks,

It might mean that, or it might mean that 

For the brand and model... I have no idea. I usually get stuff like that from Rock Auto. I have never in my life needed a new catalytic converter, even on very old cars; the only reason to replace them is usually (a) they get coated with junk from bad or leaded gas, or (b) the catalyst cracks, and you'll hear that if it happens. 

I did have something like this happen once, on a Neon, when the replacement oxygen sensor was bad. That's a pain to test because, well, you need to take the leap of faith and replace a pricey part. When I did, though, the code went away. 

I think checking for exhaust leaks makes the most sense based on what you're seeing, though. 

Some interesting info here, which surprised me. https://www.obd-codes.com/p0420