Practice questions are the best way to prepare for the exam. As long as they’re representative of what’s on the exam (structure, content, and randomness), you can use them to replicate the experience.
A very good source is over at www.projectmanagement.com. You can log into this site with your PMI ID - the one you used when you started your appliation over at www.pmi.org. This site is a sister site to PMI, that provides a hub for Project Management and PMI certificate-related discussion.
On the right-hand side, look for a link called “PMChallenge”. This is a great random PMP question generator. If you find a question that you are struggling with, take a snap-shot and bring it back to this forum, and we’ll dissect the question. This is the best use of practice questions - finding the ones that trouble us most, and unraveling them to find the source of confusion, and removing that confusion with precision.