Where do you find a poliwag on Pokemon LeafGreen?

Fish with the Good Rod you found on the Fishing Guru's brother's house in Fuchsia City. You can find Poliwags in the pond right there behind his house.

This is a list of places where Poliwags will appear as you fish with Good Rod:

Route 6

Safari Zone

Cerulean Cave

Route 22 and 23 (towards Victory Road)

Route 25 (Bill's House)

Fuchsia City

Island 2, 3 and 4