You can buy a two volume set or buy the one volume edition. Buying the two volume set is more expensive.
ISBN for one volume edition: 978-0-393-91265-4.
ISBN for volume one of the two volume set: 978-0-393-91266-1. If you buy volume one of the two-volume set, you won't need volume two until the second semester.
Check Amazon.com, Abebooks.com, Barnesandnoble.com, alibris.com, or do a good search to find the cheapest edition.
You will also need The Guns of August by Barbara W. Tuchman (ISBN 0-345-47609-3), but Mrs. McCelvey will have copies available at $7.00 per copy.