Finding the Jet Pack
It is possible to find a jet pack in each "Grand Theft Auto" game in a hidden location. To do so, first obtain a helicopter in the game (this can be done with cheat codes as well, or simply obtained in-game). Fly above the high rise buildings and find the tallest building in Liberty City. It is the one that is designed to look like the Empire State Building. Landing the helicopter here is difficult or impossible, so it is necessary to fly overhead and jump out to land on the top of the building. Enter the door on the roof to obtain the jet pack.
Xbox Button Code
The rumored button code is said to work with the Xbox 360 versions of the game which have not been patched or updated, although this has not been confirmed. To use the controller code, simply load the game and when Niko is outside press the following: Left, Right, Left Trigger, Left Bumper, Right Trigger, Right Bumper, Up, Down, Left, Right. Niko should appear with a jet pack.
Phone Number Cheat
In "Grand Theft Auto 4," most of the cheat codes are accessible by using Niko's phone. Pressing up on the "D" pad reveals Niko's cell phone; pressing up again allows access to the keyboard. Once a cheat has been entered, it is saved in Niko's phone under the "Cheats" heading. To access the jetpack using this cheat, enter the number "325-500-0100" into Niko's phone and press the call button. Niko will appear wearing the jetpack.
Mods
There are a number of mods available which make the jetpack accessible to players at any time. Modding is most popular on the PC platform, but with the right software and cables it is possible to modify game saves on the Xbox or PlayStation as well. There are modifications already available in the GTA online community that modify one type of helicopter to have the dimensions and appearance of the jetpack while still functioning as a copter would; other modifications create altered gravity settings or allow players to access the jetpack included in the game's files.