i buy used children's books for my son on ebay.
if purchased new, you'd pay 3 to 7 dollars per book, but i buy lots of them for a dollar or less there.
you have to be wary of buying things like electronics on ebay, especially refurbed or used stuff, but books are books. so long as the description says that they're clean with good bindings and no torn pages, you should be good to go.
in nomine patri, et fili, et spiritus sancti.
Meh nom eh noh...doot dooooo do do do.