In Pokémon Black and White, if a fateful encounter Shaymin is shown to a woman in Lacunosa Town's Pokémon Center they will get the Gracidea flower.
In Pokémon Black 2 and White 2, the Gracidea flower is obtained from a woman in Striaton City's Pokémon Center. The same conditions must be met as in Black and White.
Source: http://bulbapedia.bulbagarden.net/wiki/Gracidea_(flower)
Hope I helped!
