Free Shotgun Ammo

Find a parked police car, which will often spawn in the parking lots at the docks. When you get into the police car for the first time, you'll be given 4 shotgun shells. If you exit and re-enter the vehicle, you won't receive any more.

If, however, you press L+R together to start the Vigilante mission while inside the police car, you'll receive another 4 shells. Press L+R together again to cancel the mission and the game will respawn you nearby. Get back into the police car for 4 more shells and repeat the process for more ammo each time, up to the limit of 24 shells.

Never pay $1000 for a shotgun again!