I've got multiple PMs regarding the TPMS system, so I wanted to share. It's just an aftermarket TPMS system that uses special tire valve caps that measure the pressure. There are also versions that are installed inside tire (like the factor units), but why bother. The only important detail to look for is the ma pressure range of the sensor. Some of the kits can't measure beyond 70 PSI, so be careful to that. These below are good up to 120 PSI. FYI, you can set alarms for low and high pressure ranges as you wish.
http://www.carchet.com/carchet-tpms-tire-pressure-monitoring-system-with-4-external-sensors-cigarette-lighter-p3.html
I got them on eBay for $58 shipped (made an offer):
http://www.ebay.com/itm/like/401111557799?lpid=82&chn=ps&ul_noapp=true