You buy upgrades, manuals, and bandoliers at the computers that you find in the weapon shops. There are 1000 diamonds in the game, which is exactly enough to buy every single item in the computer, so feel free to buy whatever you want as long as you plan on getting all the diamonds. This particular achievement simply requires you to purchase all the upgrades for the weapons. Obviously, this also means that you have to unlock every weapon by hitting the supply convoys as the upgrades for a weapon are not available until you've actually purchased that weapon. Given the cost of buying all of these upgrades, you will not unlock this achievement until rather late in the game.