There are two required texts:
Main text: Animal Diversity 4th edition. C. P. Hickman et al. Paperback. McGraw-Hill publishers. ISBN 0072528443. An earlier edition should also be fine too.
Supplemental reading:
The Ancestor's Tale by Richard Dawkins. ISBN 061861916X. Only one edition. You can probably pick up a used copy on-line cheaply.
Amazon.com (the used section) and abebooks.com are both likely sources if you want to try to pick up an inexpensive copy of the books.