You could buy a cigarette lighter 12 volt to USB 5 volt supply, open it up, and hard wire the lighter end into the car's 12 volts. These are really cheap on eBay. That's what I'm planning on doing.
If a straight mini USB connector is awkward in the place where you plan to mount your TomTom, you can buy a 90 degree mini USB fitting, also available on eBay. Search for "Razr USB angle" (without the quotes) and you'll probably find a bunch of them for about $2 plus the customary exorbitant S&H.