Religious bookstores usually have a wide variety of these types of books in stock, and the staff is trained to help you locate the right book for age and ability.
At Gospel Supplies, there is a variety of things, even a kids section. Believe it or not I've also seen bible picture books at the dollar store, like Dollar Tree.
Probably online. There is a great collection of bible stories in Lego mode. Quiet a funny and interesting read if you ask me. Worth checking out. Try googling it, you will not regret it.
Go to a library or book search site, and search any book with keyword "bible". Look into details of book, and see if target reader age is kid's age. Also see if pictures are included too.
