Info
The Gracidea Flower is a Key Item that can be used to transform Shaymin from Land Forme to Sky Forme.
However, Shaymin returns to Land Forme during 8pm and 4am, it can also change back to Land Forme when frozen. It can be received from a blonde woman near the southern entrance to Floaroma Town. If you do not have Shaymin as the first slot of your party, she will not give it to you. Some times (If you do something wrong) she will not give it to you. Same with Action Replay, but you can get the Gracidia from it anyways.
In the movie, Pokemon Giratina and the Sky Warrior, it was shown that when Shaymin is near a Gracidea, the pollen moves and temporarily pollinates Shaymin to change it into it's Sky Forme. According to the quartily Flower Bearing of the Shaymin, that wherever the Shaymin land, a new Gracidea Flower patch grows there. Gracideas also appear in the Episode: Keeping in Top Forme!