Tire Pressure Monitoring System issues
 

My TPMS sensors are probably shot in 2008 9-5 and it looks like I have to replace them. The warming light comes on every time I start the car.
My mechanic wants $800 to replace the 4 monitors.
Does anyone have any cheaper ways of doing this.
Can I buy the parts myself (and if so, any recommendations of where or what to buy) and can I just ask a tire guy to take my tires off the rims and install the new sensors?
Or is it more complicated than that?
Thanks,

Edward