If you go into the mines near the runied house across the way from the farm place there will be three gun power barrels. Pick them all in turn and get them out of the mine. Place them on an area at the back of the ruined house that is closest to the mine. Step far back enough and shoot. BOOM! If you did it right there should be a hole in the ground. In the hole is all the ammo you could wish for!
You are welcome to come back and refil but you might have to blow up the hole again. (I have no idea who keeps filling the hole back in!)