Adventure Cycling Association. They have a really detailed set of maps that go from Vancouver BC to the LA area. Just $47 if you are non-member, $33 if you are a member of Adventure Cycling Association.
Also, they have GPS information you can load directly into a Garmin GPS unit or if you have Magellan, you can use a third party tools called “Oziexplorer”:http://www.oziexplorer.com/ that also lets you load third party maps. These are way cheaper than buying mapping information from Garmin or Magellan (although who knows how accurate). I’ve already spent for example $300 for a Magellan GPS and then $300 for various maps, so this is a real savings.