You can buy Thunder from the Department Store in Goldenrod City.
It's TM25, and $5500.
with a thunder stone
You can get the TM Thunder Wave in the Battle Frontier Shop
Trade from Pokemon Diamond, Pearl, or Platinum Version.
Bring a Pichu to Bill's Grandfather in Cerulea City where Bill lived in Pokemon Red & Blue.
no you can trade soulsilver pokemon to black
he is not in Pokemon soulsilver or Pokemon heartgold
POKEMON SOULSILVER come with a poke walker and the Soulsilver game
Pokemon soulsilver and Pokemon heartgold are basically the same game just different versions but i do recommend buying Pokemon soulsilver
SOulsilver
You can't catch bastiodon in Pokemon soulsilver.
You CANNOT LEGALLY download Pokemon Soulsilver.
You'll need to trade for it from Pokemon HeartGold. It is not avilable in the wild in Pokemon SoulSilver.