It's the same for both games but anyways...Route 20, 21, 26, 27, 41, and 47, Also in New Bark Town, Pallet Town, Vermillion City, and Cinnabar Island.
Details:
Good Rod used: 3% chance of appearing, Lv.20, everywhere listed above.
Super Rod used: 30% chance of appearing, Lv.40, everywhere listed above.
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Shellder is a version exclusive Pokemon only found in Fire red. But, Instead of Shellder, you can get Slowpoke, if you use a good rod in the Vermillion city harbor.
Shellder can't be found in the wild in Pokemon leaf green, you would have to trade it from fire red.
Go to the first city and fish off of the east side. Good rod is only 3% chance of catching one though, so it might take a while. I think you can also find them in pokewalker.
You can find it in Pokemon Platinum, Pokemon Crystal, Pokemon HeartGold, and Pokemon SoulSilver.
Find and then catch