Most satnav units can have the maps bought for them, either through the manufacturer's site or from online stores either here in the UK or over in the US.
Check out the 'mainstream' makes - TomTom, Garmin, Navteq etc. Also worth considering is buying a PDA with
integrated GPS (you don't want an external antenna one - too much hassle) - that way you can take your itineraries, ADR confirmations, etc with you on the unit too!
Preload the maps before you leave and program in your hotel/villa/condo as your 'home' address for ease of driving, and also look at the POIs (Points of Interest) on the US maps to make sure you've got all the necessary theme parks, gas stations, walmarts etc.
When you arrive in the US your GPS unit will take anywhere from 5 to 30 minutes to locate you - remember it's going from one side of the world to the other, most satnav units look first at your last known location, so it can take a while to pick up that you're in the Land of the Mouse! Best thing to do is turn it on as soon as you walk out the car rental office, so by the time you're in the car, got it on a windscreen cradle etc, there's a good chance it will already have found your location
Hope that helps!
