I've been using the Coby CA781 charger for several months now and it powers and charges my USB-charged portable devices successfully. It has two USB ports and a standard automotive 12V socket. I usually have a cell phone and an Ipod connected to the USB ports, leaving the 12V socket free for a radar detector or what-have-you. The only complaint I have is that the rubber cap intended to cap the socket when it's unused will not stay put.
I haven't had occasion to power my Kindle with it, but I see absolutely no reason why it wouldn't work. It has 0.5A current capacity for each port, so it may not charge the K as quickly as the Amazon-supplied mains charger.
Here's a link to one source that currently (pun unintentional) has it discounted to 5 bucks US. There are many other vendors selling it, including Amazon.
Larry