A house south of Lavender Town in Kanto, well after you obtain the good rod.
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In Pokémon SoulSilver, you can get the Old Rod in Route 32 in Johto, you can get the Good Rod in Olivine City and you can get the Super Rod in Route 12 in Kanto.
You either wait for a horde of them, or check where you can with a super rod.
You get it from a Fishing Guru on Route 12 at the Kanto region.
Super rod all over the place until you find a wailmer
yes, in the dragon's den. although, it wont be very strong. your best bet is a super rod