How do you legally obtain a product key

Product keys often come with a "new" copy of a PC product, in the event one purchases a product second hand or finds a product out of its wrapping then the key may be already used and registered to another place or user or just plain lost. If you do not have the product key handy and know that it has not yet been used then you can always try to call the manufacturer of the product to see if another key can be issued.