I think super rod. Anyway, that's what you need in red version, but it probably won't make any difference to blue version.
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You fish for it in the town the third gymleader is with a super rod.
You can find Shellder commonly in Route 21, Vermilion City, Cinnabar Island, Olivine City, and Seafoam Islands. You will need to fish in the bodies of water to encounter it in the wild.
Pallette Town with the Super Rod. (Yes, you need the Super Rod in Fire Red.)
You cant find Shellder in Leaf Green. You can find Shellder in Fire Red in these locations: Route 25, Cinnabar Island, Icefall Cave Entrance, Lavender Town, One Island, Pallet Town, S.S. Anne (Fish)
in leaf green you cant find ekans, arbok, oddish, gloom, vileplume, psyduck, golduck, growlithe, arcanine, shellder, cloyster, scyther, and electobuzz but you can trade them from fire red.