The TPMS sensor is the same in each tire. Majority of the time the sensor fails to send a signal to the TPMS due to a weak battery. These batteries are internal to the sensor and are not serviceable. They are rated to last 8-10 years. If you have one failing, it will be short time before the rest start failing. If you have the cash, I would go ahead and replace all (4) at the same time.
I like to stick with OE ACDelco, part# 25774006. Price per sensor varies, I have seen from $32-$50. Last set I picked up was about $130 for all four from EBay. Amazon is another good source. Last ACDelco sensor I bought from Amazon was about $40 and free shipping.
RockAuto offers a few different brands that range from $35-$55. ACDelco on their site is $46.
Make sure you pick up a new lock nut for each new sensor. AcDelco part# 15234846. They are about $5 a piece.
Also, if you are not aware, each time that you rotate your tires, you have to do the relearn process for the sensors so the vehicle knows the new location of each sensor. If not, when you get a low tire pressure warning on the DIC for the driver's front, it may actually be for the sensor on the passenger rear tire. The relearn process does not affect the sensor from sending a correct tire pressure, only where the location of that tire pressure reading is coming from.
I only say that to say you can follow the same process when getting a new TPMS sensor installed. If you do not want to pay the tire shop to do the sensor relearn with their wireless tool, you can do it for free yourself in about 5 minutes. Process is in the thread below.
http://gmtnation.com/forums/threads/tpms-sensor-relearn-matching.10242/