you can't. the only way is to trade with emerald.
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You can find sunkern in national park
Well there is only about 3 Pokemon that evolve with the sun stone and 2 in Pokemon sapphire gloom and sunkern which turn into bellossom and sunflora.
You can find a sunkern with the pokeradar in route 204 under floroma town.
You should evolve sunkern when you are in need of a strong grass type, and when you have a spare sun stone on hand. In Pokemon Ruby, Sapphire, and Emerald, you can get a sun stone in the Rocket Institution in Mossdeep City.
you have to breed skiploom and bellsprout in the daycare ..! and you have a sunskern ! :)