Shiny Poliwag can be anywhere in water, but you would have a very very very little chance of finding it.
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To find a Poliwag in Soul Silver, just use your old rod in the water in the Ruins of Alph. The first time I put my rod in the water, I caught one! :)
Unfortunately the chance of finding a shiny Pokemon are quite slim, either your lucky and find one, or you trade with someone who has a shiny you want.
You can't catch a poliwag in Pokemon Sapphire. You have to trade from Pokemon Fire Red or Pokemon Leaf Green.
you find the sihny ston in mewtoo's cave
I'm sorry, but you will have to trade from Pokemon LeafGreen or FireRed. There is no way to find a Poliwag in the wild in Emerald.