I bought mine from billc1950 eBay seller. What a great guy to deal with too. Great comms. He even threw in a spare valve cap for free. He is what eBay should be all about. The price of these was $2 each plus about $2 for the postage.
This is his blurb for the caps.
Brand new Safety Monitoring tire valve stem caps let you eliminate the guesswork when it comes to your vehicle's tire pressure. These quality metal caps fit on any standard size existing tire valve stem. Visually alerts you when the tire pressure is low to help avoid premature tire wear or blowouts from under-inflated tires.
The cap shows three different colors. Green shows that the tire pressure is normal. Yellow shows that the tire is low by 4-5psi. Red alerts you that the pressure is down at least 10 psi.