Usually you can find them at the rest stops and sometimes they have the surrounding states too. If you have time you might even be able to write each states DOT or even order them online at ther websites and they might be able to send them to you. Also I have seen road maps/atlases for sale at book stores like Barnes And Noble and places like Staples, Office Max, and BJ's. Also you can buy them directly from Rand McNally at
https://store.randmcnally.com/2019-road-atlases.html. Instead of tearing them out of the atlas you could always make a copy of each state you will be traveling through.