This has been covered before.

http://wiki.maemo.org/N900_Accessories#Car_Chargers

Any standards-compatible microusb charger should work. The N900 follows the standards that require the data pins to be shorted in "dumb" chargers. I purchased a no-name brand car charger online and it works fine.

@jschan: The n900 hardware and software manages the battery charging, the charger just supplies constant power.