Whenever I was keeping an eye on my lil cousin we'd go to the public library bc it had a great section of Japanese manga and American graphic novels (usually mixed together) in the kid's/teens section and we'd just sit there and read. I haven't done that as much now that cuz is grown up. tbh I'm not a huge fan of manga, even of stories where I like the anime.
The only manga I thought was truly great was Miyazaki's "Nausicaa", which I ended up buying and reading multiple times. "Yotsuba" and "Azumanga Daioh" were pretty entertaining too.