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.
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Rock Slide is not a TM in Leafgreen. In Leafgreen Rock Slide is a Move Tutor move and the Move Tutor is hidden somewhere in Rock Tunnel.
Aggron doens't learn rock slide in this version
not, you will need to use tm80, rock slide, on onix.
If held by a Pokemon it will increase the power of rock type moves such as rock slide or rock throw. Giving it to a rock type Pokemon is highly suggested.
Rock Slide is taught by a move tutor in Rock Tunnel.