If you were to compare our world to theirs, I would think that most of the ammunition would come from sheriff/police stations, gun stores, houses in which people stocked up, supermarkets, military armories/bases, gun ranges. If you think about our world, there is ammunition everywhere. While there is a also an abundance of food everywhere you look in our world, in TWD world most of this food has spoiled, eaten by animals, etc. Ammo doesn't really go bad, so it is still abundant especially since there are fewer of the living to fire a weapon.