You can find the mod and installation instructions on community sites like GamesMods.net .

The is a vehicle modification for Grand Theft Auto V that introduces a specialized FIB-themed version of the Stockade armored truck as a standalone "addon" vehicle. Mod Details Version: 1.0 Size: 2 MB Upload Date: January 31, 2019

FIB Stockade [ADDON] 1.0 – All Mods – Grand Theft Auto V

Add-on (meaning it adds a new vehicle to the game rather than replacing an existing one) Where to Find It