No way to get it Ingame, though you can get it 4 different ways. Here they are...
1.Do the simple thing and trade from Diamond/Pearl/Platinum/HeartGold/SoulSilver
2.Get it over the wifi room with a friend.
3.get it from a friend or a random person over GTS Negotiations
4. In black and white, there as an event to get these beasts in shiny form, trade from your black or white game to get a shiny one.