Plant Maintenance certification questions are taken from the official SAP PM training courses. SAP provides some excellent training on how to find the correct certification course at https://educops.sap.com/demohub/demo/taprep/launch.htm. At that link you'll find
Part 1 - Introduction, Which Exam is Right for You
Part 2 - Certification Finder
Part 3 - Exam Contents in Detail
Part 4 - Define Your Learning Gaps
Part 5 - Supplemental Materials
Part 6 - Study Tips
Part 7 - Frequently Asked Questions
Somewhere in this training you should find where to find some sample study questions. If you google SAP PM certification sample questions, you will probably find many examples. You'll also find folks trying to sell you what they believe are actual exam questions. I recommend staying away from those unethical web sites like the plague. In all probability they will charge you money and give you questions they made up themselves which will not help you pass the exam. Unethical is unethical, I can't figure out why anyone would expect to actually get real exam questions from anyone advertising stolen exam questions!
If you're in Asia Pacific, you'll need to take a full set of training classes from one of the Authorized Training Partners in order to sit for the exam unless you have 2 full implementations and an authorized implementation partner to sponsor you. SAPTechED may be an exception, though. I'm not sure about that.
If you're anywhere else in the world, then you can just register for the exam through an authorized test center (PearsonVue in the US and maybe elsewhere) and take it at your convenience. The certification questions are taken from the class material, so, while not impossible, it is strongly recommended that you take official SAP classes from somewhere and study the class material thoroughly (3 times or more all the way through) before sitting for the exam.
No matter where you are, if you're interested in taking training from SAP itself, check the main SAP Training site,http://www.sap.com/training-and-education/index.epx.
Hope this helps!