First of all, you have to have all three islands. And you should have at least 50 hidden packages found but that is not required. Go to your hideout in southside hoods in shoreside vale (the third island). And collect the weapons, then, get a car and drive around the back of the building where you got your weapons, you should see an opening and there is a truck. Go towards the opening but don't go through. Then turn around (you should still be in your car) and go back to where you got your weapons, there should be more for you. You can get a lot of bullets and bombs in a short amount of time if you repeat this. And if you get 9999-25 bullets for a gun, you get unlimited bullets for that gun. I don't know if you can get unlimited bombs for a certain bomb.