>In Pokémon Mystery Dungeon: Explorers of Sky, a Gracidea can be obtained once per day from one of the citizens of Shaymin Village after the player recruits one of them. The Gracidea can only be used in a dungeon, and upon leaving the dungeon, Shaymin reverts to its Land Forme. A bow resembling a Gracidea also appears on the Sky Gifts found in Sky Peak.
This Gracidea is then used to change Shaymin's form.
>I found out from the official strategy guide they have at my local game stop that there are a number of dungeons that if you take Giratina into, It will be in Origin Forme. I know that Destiny Tower and Zero Isle. Central (New Zero Isle. Dungeon) are two of them.
Source: 1 and 2.
Hope I helped!