After you beat Blue, the 16th gym leader
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Surf west of Cherrygrove, use rock climb and headbutt the trees. Catch it.
come back to cinnibar island with a Pokemon with rock climb n climb the rock until u find it
Anywhere you see rocks going upwards (to check if you don't have it go to something if you think that's it, then hit A if nothing happens you can't climb it).
To get rock climb in HG/SS you have to beat Blue's Pokemon gym in Veridian City first. Then, go talk to Professor Oak. He will give you the HM rock climb. Note: If you trade a Pokemon from D/P/P , even if you taught it rock climb, you cant use it until you beat Varidian gym.
You'll get Rock Climb from Prof. Oak when you have defeated all the Gym leaders in the Kanto region. Prof. Oak is at the laboratory in Pallet Town.