I'm on the last chapter in the game, have maxxed my salvage rank and dev level for every city (except one). I can't seem to find a place to buy premium cylinders. I've heard that they can be bought before the last chapter and not at an A rank salvager too. It seems the common denominator is that you stop being able to buy them at some point, but I can't confirm that since I've never been able to.
How do you buy premium cylinders?