Go through Diglett's Cave and you'll end up near pewter city, where Oak's aide will give you HM05 Flash if you have caught 10 Pokemon, he's located in a building south of the cave. After obtaining HM05 go back to Cerulean City, go through the robbed house, head east and cut a bush with various trainers, defeat them and head South to find Rock Tunnel, use Flash to get passed the tunnel and you'll be in Lavender Town.
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talk to officer jenny after getting the thunder badge
Power plant
You can get it in Cerulean but pikachu won't let you use it on him in yellow.
Get the Thunder Badge And Talk To Jenny And She Will Give You One
A Thunder Stone is an evolutionary stone that allows certain Pokemon to evolve when exposed to it. In the third generation of Pokemon games, there is one Thunder Stone in New Mauville, and they can be had from the Diving Treasure Hunter for Yellow Shards.