If you are driving, and not using a GPS, then one way would be to mark a map with your route and put a dot at about 15 mile intervals along your route (good to put the dot at some landmark like a crossing road that will have an exit, so you can tell where it is when you are driving. Then write down the zip code of each dot. When you pass the dot, enter the zip code on the scanner.
You can get the zip code for a location by zooming in on the location in Google Maps, then clicking the point. A box with location information including zip code should show in the upper left corner (http://maps.google.com).