Cerulean Cave would probably be the best place to catch high level Poke'mon, but if you catch a high level Poke'mon it won't be as strong as one you raised.
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From Battle Frontier and Victory Road.
You slather honey on a golden brown to attract pokemon. Do this until you find a cherubi. Then level it to level 25 with high friendship during the day.Hope this helps!
Victory Road has fairly high level Pokemon in it
High level Pokemon.
you can find it in cinnabar's destroyed mansion. Also there are high level ditto's in cerulean cave.