If you have pokemon diamond or pearl and you're going to purchase pokemon platinum, pokemon is having a special event feb. 8th to the 14th. Take your game to toys r us and you can recieve a legit shaymin that can be transfered to pokemon platinum.
The unique thing about the event Shaymin is that it is the only Shaymin that will work with the new item Greacidas Flower to change into Sky Forme. The only way to get the flower and the compatible Shaymin is through the Toys R Us event. The same thing holds true for Regigigas. It is the only one that will unlock the chambers for the three Regis in Platinum.
There are two ways to obtain the Gracidea Flower in Floaroma Town. One, you could get the Lv. 50 event Shaymin. Or, Two, you could get a wi-fi event Shaymin at Route 224. If the Shaymin is originally from Diamond or Pearl, you cannot receive the Gracidea Flower.