Go to this site and send them an e-mail with your full VIN. Ask for the part number for Bank 1 Sensor 2. This is the post cat sensor.
I can tell you that you're looking at around $100 from the dealer and $60 from the link below.
Also in the troubleshooting tree for DTC P0140, it goes all the way to step 15 before saying to replace the o2 sensor!!!
Also if you have two post cat sensors you may try swapping them from side to side and clearing the code to see if it stays with the sensor or bank 1.
Good luck.
http://www.gmpartsdirect.com/