All exam questions in my courses come directly from the textbooks so you will absolutely need a text for the class. The thing is, previous editions of a textbook can cost substantially less than even used versions of the current edition. So, while I can't promise that older editions of a text will prepare you for each and every question on an exam, if you need to save money on your books this semester you might consider going for an older edition. (Please note: this advice does not apply for PSY 101 students or SOC/PSY 261 students- both of these require online supplements that only come with new books.)
Keep in mind though, the more editions you go backwards, the more likely it is that the material will have changed substantially. My suggestion is that you only go back two editions at the most. (EX: from 10th to 8th).
I've provided you with links to some sites with excellent prices on used/older textbooks below.