>The Gracidea (Japanese: グラシデアの花 Gracidea Flower) is a key item introduced in Pokémon Platinum that changes Shaymin between its Land and Sky Formes. -Bulbepedia
Unfortunately, you can't get one in Diamond, as it was introduced in Platium.
In floaroma town first buliding you see when you enter talk to lady and you get gracedia have shamin in your team
but in platinum sorry