The safari zone has some. There are swarms of different Pokemon that come each day, you have to listen to the Professor Oak's talk show and he will tell you what Pokemon has come that day and where. Trade in from Diamond and Pearl.
If you have Soulsilver, you need a Pokemon that knows the following moves: Flash Surf Whirlpool Sweet Scent Go to where you caught Lugia and then use sweet scent. If you're really lucky, Arceus will come.