40's can be bought from the clerk at the 11-seven at all times of day. They yield +2 Charm and cost $90.
Fighting
In addition to buying them, a 40 is given to you for defeating one of the Eastside mob members on the third island, armed with a katana. There is a glitch, however, where it won't give it to you, and will instead just give you the money and strength that would normally be granted in a fight.
Quest
There is one use for 40's other than simply using them to get more charm. The quest to get a tier 1 flamethrower requires that you bring a 40, a leafblower, and a lighter to Pyro Perry on the Fourth Island, though this mission is only available in the Director's Cut of the game.
