I don't know where you are, but if you're near a good bookstore, they can order multiple volumes and can often get discounts for education. You could run an ad in a local paper (unless that violates school policy) asking if someone in the community can donate them. You may be able to work something out with Amazon.com -- see if they might consider discounting for volume purchases for education purposes.
Ask your school librarian to do an inter-school search or request. Often times a neighboring district or even one within a state education dept.'s region online services will enable you to borrow entire class copies. (Ie, in teaching a distance learning SF class, I am often dealing with 3-4 schools many miles away. The librarian often assists when I need titles or number of volumes unavailable in my own department. It takes a bit of searching, but usually after a day or so the necessary number of books are located. Then it is a matter of shipping.
If you have a shared services transport van between nearby districts or counties, you may find this an easier transaction that it sounds!! Good luck. Its a great novel to teach and a regular on my classroom reading list.
