The ANT+ network key and ANT-FS network key are available here: Network Keys
Note that you need to be signed in as an ANT+ Adopter to access these keys, and that they should only be used on ANT+ devices (i.e. devices that comply with an ANT+ device profile). If you are simply using ANT to transmit data in a way that you have defined, then use the public network key which is programmed by default into ANT devices.
For more information, see: ANT+ Basics