Since Claris no longer sells or supports version 6, abandonware sites and vintage software repositories are your friend. Look for a .toast or .iso image of the original CD. The installation code is usually a string like: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX (often found on a text file inside the archive).
Each standalone retail copy of FileMaker Pro 6 licensed a single user to install the software locally. filemaker pro 6 installation code hot
The Legacy of FileMaker Pro 6: Understanding the Software, Installation, and Modern Alternatives Since Claris no longer sells or supports version