The Source (in Canada) has a 90W DC car power inverter on sale until Feb 8/2012 for $19.99 and it has a USB plug in it too. Use your standard TouchPad barrel adapter with this when on the road.

Here's the URL (with photo): Nexxtech Fusion 90W DC Inverter | Inverters | Ratings & Reviews | TheSource.ca

Here's the sales pitch: Conveniently use the DC battery power in your vehicle and convert it to AC power for your electronic devices with the Nexxtech 90W DC Inverter. Simply plug the power inverter into the DC outlet of your car and connect your electronic device via the AC outlet for instant use or fast charging. The Nexxtech DC Inverter also includes a USB port for devices with a USB connection including cell phones, MP3 Players, PDAs and more. This slim, lightweight inverter provides 90 Watts of continuous power and comes equipped with overload temperature protection.