I could have written this post a year ago. Not only did that happen to me- but it was the same authors! I had Marieb for I and Tortora for II. Our instructor said don't worry, I could use Marieb, but I found that there were some differences in terminology and in the illustrations. Because my instructor used the illustration for our lab practicals, I felt lost and disadvantaged!
So, I decided to buy the Tortora book, but instead of buying it new at the bookstore (very expensive$$$) I went to Amazon
.com and bought an older edition for $20- and it worked PERFECT!!!! It had all the same pictures and same terminology. The only real difference was the page numbers and I think there were some slight changes in the pictures, but nothing significant!
Good luck and don't fret there are cheaper options!