After defeating all 8 kanto gyms, go in the cerulean cave. Search everywhere and you will find it.
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You get it from The Power plant use your item finder thing its by where you catch Zapdos.in cerulean cave
put in Pokemon fire red and there will be a elekid with an electrizer in the valley wind works field.
mt mortar
You can find the Electrizer in Cerulean Cave after you defeat all sixteen gyms and battle the elite four for a second time.
buy it at the rare Pokemon store