Anyone who purchases a Symantec product, including Norton Anti-Virus software, receives a product key as part of the purchase. "Free" product keys are typically those that have been hacked or stolen, so be wary of these.
The product key for your Norton Anti-Virus software is located on either the license certificate or version upgrade notification that came with your software. In the case of digital purchases, Symantec may have sent you an email containing the product key. Not all Symantec products require a product key. To activate your software, you might instead need to use an activation code or serial number.