Yes, you can get a Gracidea Flower in "Pokémon SoulSilver." In Soul Silver you have to go to Goldenrod City's flower shop (with traded Shaymin not Action Replay Shaymin) and talk to one of the sisters there.
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It is impossible to find the Gracidea Flower in Pokemon Diamond and Pearl. It is available in Platinum.
If I'm correct, the Gracidea flower cannot be obtained in Pokemon Diamond. Sorry! (Note: The Gracidea can be obtained in Pokemon Platinum.)
you can't you can only get it on Pokemon platinum
The gracidea flower item cannot be obtained in Pokemon Diamond, since Shaymin's Sky Forme is only available in Platinum and later games.
You can't. It is specific to Platinum.