I think there has been an event or there is one at the moment the only other way is using an Action Replay Cheat System.
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Shaymin cannot be found in Soul Silver.
You can't change Shaymin into Sky Form in Pokemon Diamond (that includes pearl). You can only change Shaymin into Sky Form in Pokemon Platinum (that includes Heart Gold and Soul Silver). You can get the Gracidea to change Shaymin into Sky Form.
You can't find Shaymin directly in Heart Gold, you'll have to trade your Shayim from Pearl/Diamond/Platinum onto your Heart Gold/Soul Silver.
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.
sorry you have to trade it to Pokemon soul silver