The extended ammo pouch is the best ammo holder in Far Cry 3. To craft it you will need one Golden Tiger Leather. To obtain this leather, you first need to get the Path of The Hunter quest that asks you to kill the golden tiger with a bow. This quest is unlocked after liberating the Mosquito Yard. The Mosquito Yard is directly South of Amanaki Town.
Pro Tip: Have at least two weapons going into this because you will need to swap one for the bow. If you see a regular tiger in the area, kill it with a gun - not the bow. It takes three shots to kill a tiger with a bow and you don't want to be messing with more than one tiger at a tiger.