newer xm and sirius units run off of 5v, so if you have one of their cigarrete lighter power adapters laying around u can use it to power your first choice.
Or if you have a cigarette lighter sony psp charger, or a cell phone charger that uses mini usb (moto razr, most pocket pc's), or a portable navigation charger that uses mini usb u can use one of these. Reason being is all these chargers use a 12v to 5v step-down transformer.
But if you don't feel like dealing with transformers, you can spend the extra money and buy the 12v ones.