i would use a super rod. if this is the gen 1 games (red yellow blue fire red or leaf green) you can catch him where the SS anne was (i think you can only catch him on blue or fire red but im not positive)
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Super Rod.
Use a good rod in seafoam islands.
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.
You can catch shellder in the museum at Oreburgh city,,, to catch it you must go through a secret passageway in the back and you ill find a secret garden with shellder's and eevee's. It don't work wheres the secert passageway.!!!.!!!?!!! There is no such thing. You get a shielder on route 205 with a super rod, and eevee are found in the Trophy Garden or in front of the mansion.
Fish with the Good Rod or Super Rod at New Bark Town, Union Cave Route 26, Route 27, Olivine City, Vermilion City, or Pallet Town.