You must get it from an event that has already expired. If you missed the event, you can get the mystery gift event card from someone got yellow forest, and kept the card.
in the yellow forest
the Viridean Forest is close to Viriden City in pokemon soulsilver
Viridian Forest
it is in the forest in jhoto
in the Ilex Forest
To get a Pikachu in Pokemon SoulSilver, you must go to Viridian Forest, which is in the Kanto region.
Yellow Forest Wi-Fi Connection April 1 - May 5 2010 Heartgold or Soulsilver only
Yellow, the character that you would face at Mt. Silver, is not in Pokemon SoulSilver. However, that same character is there, but with a different name: Red.
viridian forest
Ilex Forest
There is no such thing as a Yellow Forest.
Headbutt is no a HM in Pokemon soulsilver. However a man in Ilex Forest will teach it to any Pokemon that can learn it.