YesAsia is a good resource. I'm not sure if they have exactly what you are looking for, but here is a place to start:
YESASIA: Children - Books in Japanese -- - Free Shipping - North America Site
There are links for elementary and educational and, reading and speaking and tons of stuff, middle school and high school levels too and tons of other stuff. Sometimes it gets expensive, but if you can at least find a title or two you like you can search for them in other places too. Hope you can find something good.