You must trade from LeafGreen version.
You cannot {unless you trade or hack}
what you have to do is catch a slowpoke, then find a king's rock and make your slowpoke hold it. get another copy of emerald, or ruby and Sapphire will work fine, and then trade your slowpoke, and watch it evolve into the magnificent slowking. ENJOY YOUR NEW POKEMON
You can find slowpoke near Azalea town.
Slowpoke is only available in leafgreen.
You can catch Slowpoke in Pokemon Gold in the Slowpoke Well that is by Azalea Town and you can also find them in Tohjo Falls.
You can find slowpoke in pokemon white if you go to the Abundant Shrine, and use surf on the water. When you see a ripple in the water, go to it. Sometimes the wild pokemon is a slowpoke. I have caught one before like this, it works.
Well in Azaelea
There is no Pokemon mansion in sapphire.
He is not in Pokemon: Sapphire. You will not be able to find him. He is in Pokemon: Diamond, Pokemon: Pearl, and Pokemon: Platinum. You will definitely have trouble finding him in Pokemon: Sapphire.
King's rock is an uncommon pickup item. I know of no other way to get one- but I have four. I really don't find them very useful- as there is no Slowpoke in Sapphire. Just find a Pokemon with the Pickup ability, and proceed to tread grassy areas or deserts. Per Below. http://bulbapedia.bulbagarden.net/wikiPick_Up
you can't find it in Pokemon sapphire, but you can find it in Pokemon ruby. Hope this helped. :-)
leaf green only.