From someone who's tried finding past AP exams...
If you're having trouble finding them, don't worry about it. Understanding the material is more important than getting the exact questions in hand. For this reason, I recommend that you get an AP Physics review book and do all the practice questions/exams in that. Doing so should be an excellent indicator of how you will perform on the AP exam.
If your goal is to get a 5, working from review books should be more than enough for your purposes. Of course, if you have a real test or two, it doesn't hurt to take them as well.