The Windows 7 Ultimate product key is located on a card inside the box that the installation disk came in, as of 2015. Computers with Windows preinstalled should have a product key sticker on the underside of the computer.
According to Microsoft, a lost product key cannot be replaced. Users must purchase a new copy of Windows 7 Ultimate and use the new key provided to register the software. The Microsoft store no longer sells Windows 7 Ultimate, as of 2015. Windows 7 Ultimate is available for purchase on Amazon.
Sites that claim to have free Windows product keys are typically unauthorized and may be scams.