There are many places to find one, but the easiest to find is the one in Victory Road. Also on some days you can buy some at the Pokeathlon with your athlete points.
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after you beat the gym
you have to use hm rock smash on rocks to find them.
you find him in the ruins of alph by weeds and you can find smeargle on there same thing on Pokemon heart gold (natu evolves at level 25)(smeargle no evolvetion)
just run in the grass
near safari zone were the grass is