I don't have a sample at hand, but a simple form should do the trick. It should ask for the teacher's name and grade, a list of items to be purchased, approximate cost, description of how they will be used, and a place for the teacher's signature and date. You should also include information about how to hand in the form (ie place in the PTO box in the office, etc.) and who to contact for more information. Appoint one person to manage these requests, and steer teachers to that person when they want more information, etc. Also, just as an FYI, you should be requiring teachers to provide receipts for whatever they purchase. It's important to have those for your financial records.