I got 3 of the Motorola Vehicle Power Adapter micro-USB Rapid Rate Charger from Amazon when they were under $4/piece. They all worked fine with my n900 before it died, and they are all working fine with an n8 now. With anything that cheap though, you should be careful about overcharging as not all chargers are created equal in terms of maintaining the quality of your battery.

I don't know what it'll be yet, but but Amazon lightning deals also has something involving charging micro usb devices today at 3pm est (though it doesn't sound like a car charger):