Without the use of a Gameshark or an Action Replay device, there is no rare candy cheat for Pokemon Leaf Green. Rare candies can normally be found in hidden areas in the game or through battles with the assistance of Pokemon abilities.
Pokemon Leaf Green is an enhanced remake of Pokemon Blue, one of the two original Pokemon games that contain a rare candy cheat. It is possible to duplicate rare candies in Pokemon Blue without the assistance of a cheat device by exploiting a glitch Pokemon called Missingno. This glitch was purposely removed by Nintendo from future Pokemon games, as it has been shown to have adverse effects on the game.