I don't know which bank is which side of the car off the top of my head, but yes sensor 2 would be the downstream sensor (after the cat). Hopefully the issue is just your sensor, and not the cat itself. As in, is the sensor doing it's job and telling you that you have a bad cat. Nevertheless, I would still try the sensor first, much cheaper and easier.
Replacing: Easy to get to, and easy to install. Getting the old one out might be a bit of a pain depending on condition. Just be careful, and use either an 02 sensor socket, or a high quality wrench.
As for parts... The GM part number is 92068713, and here is the part: https://www.amazon.com/ACDelco-213-4...ords=o2+sensor
Or you can get this one and save a few bucks: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I installed 2 of the NTKs about a month ago with no issues. Looks identical to the OE part, even comes with the anti-seize on it already.
2009 PBM GT
Under the Knife Again
Wiseco Pistons, K1 Rods, Johnson Lifters, BLP Keep Hatin' PDS Cam, Roto-Fab, BLP Ported Intake and Throttle Body, Kooks 1-7/8 long tubes, XForce Varex Catback, ZL1 Converter, Camaro Trans Pan/Filter, GXP Brembos
Breyton GTS-AV Wheels, 6k HIDs, Holden Steering Wheel and Paddle Shifters, Eibach Springs, 35% Tint
Tuned by Day Tuning