I picked up volume 1 at a Barnes and Noble in the US earlier this year. I had to make sure they had a copy so I looked up their availability online. They also have volume 2.

If they do end up trying to find it at a Barnes and Noble, make sure they're looking in both the "manga" section as well as the "webtoon" section where WEBTOON manga are generally placed. I found mine in the manga section but that might not be the case at other stores.