Boxed Children's Books
You can order a box of books from this site for $12.99. They will range from toddler to young adult books. Each box contains approximately 150 books (give or take). You can also order more age/grade specific books, but it costs a bit more. There is now an advisory on their site that due to high demand, boxes may not ship for up to 5 weeks, but for $12.99, that's a small price to pay :)