Cherish Ball isn't accesable to be able to actually use on a Pokemon in any pokemon Game.
Cherish Ball's are the event pokeball's that Special Event pokemon that are Given to you, will be caught in.
The only way to get a Cherish ball that you can use in your game, would be to use an Action Replay, Gameshark, or some other form of Hacking/Cheating in your game.
In a Review,
Cherish Balls aren't found in your game
The only time their present, is if the pokemon in them, was Distributed through an event.
Have a nice Day :]