You may have that already. It might come with the installation disk of your software, or you might have been given it from your provider. The product key has 25 characters divided by dashes.
It will look something like this: 1234-5678-0000-abcd-9101-xyz1-a.
If you believe a product key is already installed on a workstation but cannot recover it, Silver Bullet has a utility for recovering installed product keys. Please send requests for the utility to Support@sbullet.com