Is there a way to save my students money on a textbook?

An easy thing to do that can be very helpful to your students is to put a copy of the textbook (or two or three copies) on reserve in the university library. Students can then photocopy critical sections of the book (e.g. the homework exercises.) This is particularly helpful at the start of the semester when students are waiting for copies of the book that they've ordered online to arrive.

If you have control over the choice of the textbook, you should consider moving to a cheaper book or even an open educational resource (OER) that is completely free to students.

I have found, by direct experience, that publishers are sometimes willing to offer steep discounts. At any rate I pulled this off once, and arranged for my students to be able to buy their book at an approximately 40% discount to what was available on Amazon or anywhere else.

This involved the students buying their books directly by mail from a private page on the publisher's website, and this led to an ugly argument when the bookstore's manager found out about this. I ended up having to read my faculty manual closely so that I could call the manager's bluff. In the end, I (and more importantly my students) won.

This could well work for others, and without the argument. :) But the bottom line is that publishers will negotiate. "I'm considering requiring a textbook..." are precisely the magic words. Just look up the contact information for the publisher's regional sales rep on the Internet, and call or e-mail them.