If the textbook is published by CSLI:

Pick up a copy of the text from CSLI Publications in Cordura Hall (CSLI).

If it's published by someone else:

Requesting free copies from publishers

You should order copies EARLY. Check with the faculty member you're TAing for about 2 months before the course starts to see if there is any way you can help in ordering a desk copy (this is the best way to ensure you have one when the course starts). If you have difficulty getting a free desk copy from the publisher, go see the Student Services Coordinator (Alyssa Ferree, aferree at stanford dot edu).