get all 16 badges then talk to Prof. Oak
Chat with our AI personalities
Brock gives it to you after you beat him in kanto
in soul silver
Rock Slide is TM80, which you get from defeating Brock. Unfortunately, that is the only one you can get. Consider Stone Edge from Battle Frontier as an alternative.
by rock smashing rocks at ruins of alph
You have to beat all 16 gym leaders to get rock climb I believe.