You mentioned Barnes and Noble in your OP, but that is definitely where I bought ours last year. Walgreens has them sometimes too.
You may need to find a clerk to help you locate them.
Edit to add-for folks who think they are unnecessary; if the kiddos are in the car, learning how to read a map on the fly is a useful skill. It also gives them something to do that isn't a screen. Think about who made those maps and how. Lot easier to travel without a paper one these days, but an important part of history.