Sunkern is rather Uncommon, but if you use a Poke Radar, you can go to Route 204, preferably at the Northern side.
There is no 'platnium', but you can find it on wild pokemon.
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No, you'll have to trade it from Platnium. Which in Platnium you find it in the Rock Peak Ruins.
in girantinas cave
Honey Trees
in the trophey garden
you can find one with a traner on 216
beat every trainer
use your map to find out
you have to trade him from Pokemon platnium
U have 2 trade him
go to the suburban route on the pokewalker.