The Prison Bus is exclusive to the downloadable episodic pack The Lost and Damned for Grand Theft Auto IV. The prison bus is very different to the normal bus as it is shorter, faster and heavily armored for prisonmate transportation and protection.
The bus appears in the mission Off Route where Johnny must hijack it full of inmates, lose a three-star wanted level and then drive it down to Acter Industrial Park. It disappears after the cutscene. It also can be seen in the mission Get Lost when the player must break into the Alderney State Correctional Facility and then when he is breaking out. There is only one way to obtain this vehicle and that is to purposely kill all prisoners using sniper rifle and then yourself by using a rocket launcher at your desired Safe House and you have to be some considerable distance away from the Prison Bus to prevent you from destroying your Prison Bus, However you will have to do this fast because you will be redirected to a different location on the map. This also makes you fail the mission but you do as always you will receive a text from Stubb's saying "Start Off Route again". If you was to back to that location you will see your very own Prison Bus at your Safe House location.