When on a road trip diapers can be bought at any grocery store, Walmart Stores, Dillon's etc.
In just about every state you'll pass through you should be able to locate a Walmart Store.
Walmart will have diapers.
Dollar General also has diapers and they are in most locations as well.
When traveling you should bring at least a box of diapers with you to ensure you have plenty.
I always keep a box of 120 diapers in the Van we travel in so I have plenty of diapers to change my baby with.
If we're gone on a month road trip then that box will usually last the entire trip.
However if not I can stop at Walmart and get another box of diapers and have some left when I get home.
But you're going on a few days road trip then taking a few days worth of diapers with you on the road trip should be enough.
Then you can stop at Walmart or other stores to buy some more diapers if needed.
You can also have diapers shipped to a hotel or motel where you are from Walmart.com or Amazon.com