Non-Free Product Keys. For those who require a non-free Microsoft Office product key, there are authorized sources where you can purchase a valid key. These include: Microsoft Store: The official Microsoft Store is a reliable source to purchase genuine Microsoft Office 365 product keys. They offer different packages and subscription options The Product Key for Office is on the CD case on a sticker that says something like "DO NOT LOSE THIS KEY". The Product Key for Office is not on the sticker on the laptop. If you have lost your XP product Key , the best option is to call Microsoft. Call on Microsoft Licensing Fulfillment Center Toll Free Number (800) 248-0655 Monday through glRy8.