You have to use your poke radar when you beat the elite four at routes 225 and 226.
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you should look at serebii.net for that answer.
The route west of Viridian city. You can also get a mankey in rock tunnel
Mankey evolves at level 28 into Primeape.Mankey evolves into Primeape at level 28.
In Route 9, 38, and 39.
You can catch one at Route 225 or at Route 226 (using the PokeRadar).