How many copies of the books each school needs will vary. You will need to calculate how many children will read how many books how quickly.
Books can be sourced in a variety of ways. In Singapore there is Kinokuniya, Bookaburra, Littered with Books, Popular Books, etc. Online you can order books via Book Depository (UK) and Follett Titlewave (USA) -- which are more economical thanAmazon (which charges shipping per book).