A fun little project. You can get some cheapo cig-lighter/usb charger from Wallyworld, and salvage-out the guts. Normally not very complicated, just a tiny 5v/500mw regulated power supply chip and the USB socket, usually on one tiny printed-circuit board. Additional features could be an LED indicator and on/off switch.
Don't let it intimidate you, it's really simple, think of it as wiring in an indicator bulb, just +12v and GND.