Castform can only be found on the Pokewalker route "Treehouse" after walking enough steps to unlock it and obtaining the National Pokedex.
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In the tree house route on the poke walker
No, you can not catch Phanpy in SoulSilver. It is a version exclusive to HeartGold.
there is a mew in Pokemon soulsilver but you need to catch all leadndery Pokemon including event Pokemon
With a pokeball
Route 44.