The jet pack, otherwise known as the Mammoth Thruster, was made available by Rockstar to buy and to use in the Doomsday Heist in GTA Online.
It is also available in GTA 5 story mode, although you will need a mod to spawn it.
There are individual jet pack mods available, but we would recommend installing the Enhanced Native Trainer as not only will you be able to spawn the jet pack, but also you’ll have hundreds of customisations at your fingertips to completely transform GTA 5 story mode (not GTA Online).
To install the Enhanced Native Trainer, you can follow the instructions in the video below.
Once installed you go to Vehicles > Vehicle Spawner > Emergency & Military > Mammoth Thruster and then you can spawn as many as you need and want.
If you want to see them spawn randomly in the game, then you can install the Traffic & Vehicle Randomizer mod and if you’re lucky you’ll see the cops using them and they can even spawn on the helipads around the island of Los Santos. See both in the video at the top of the page.