I already have one but I want another one.
You cant catch it in any game. You can only get them from events. There are currently no events for it so you're out of luck but thats why they made GTS so try there. Either that or you could just wait for an event but if there was an event, it would most likely be for B2/W2 not platium. So if you have B2/W2 you could wait for the extremely low chance of a jirachi event but it would most likely not happen. Source: http://bulbapedia.bulbagarden.net/wiki/Jirachi
