I always found, especially in my smaller classes, that I could just go to the instructor and say "Hey, I've got an old edition. Can I copy your book to get the problems?" Far more often than not, that was an adequate solution.
The last time I was in school, I ended up getting three different editions of the same textbook for a class. Initially, I got the previous edition of the book, which the teacher said I couldn't use because some of the problems were different. Then I got the "international" (meaning Indian) edition of the book, which I couldn't use because the teacher said they had replaced a few of the problems with Indian-specific problems. Finally, I ended up getting the overpriced new edition. This gave me the chance to see what the differences really were between editions, though. The differences between the new and previous editions? They combined two similar chapters and replaced the last paragraph of the earlier chapter and the first paragraph of the later chapter with a single paragraph. They also re-numbered the problems at the ends of the chapters, replacing a few of them with new problems. Finally, they added a paragraph in the introduction boasting about how improved the new edition was. The difference between the international and standard editions? They re-ordered the chapters and replaced American terms with Indian terms for a few of the problems.
I wonder if part of their "justification" is that the rearranging and rewriting of the questions helps to prevent plagiarism?