You can't. Deoxys Only comes as an event Pokemon in Pokemon Ruby, Sapphire, and Emerald. You would get a ferry ticket to Birth Island to catch it. There was also an event for Pokemon Diamond and Pearl for a Lvl. 50 Deoxys from Gamestop. Hope I helped out a bit!
there is no spaceship area but if you are wanting to change the form of your Deoxys it is at veilstone city . Just have your Deoxys in your party and click a on them. I hoped this helps if you have or will ever have a Deoxys in Pokemon diamond pearl OR platinum :D
neither of them is better, they are both the same, but in diamond, you catch dialga, and in pearl, you catch palkia, and you can catch different Pokemon in each game for example: you can catch this Pokemon in diamond, but not pearl but: you can catch this different Pokemon in pearl, but not diamond
You can catch Deoxys at Birth Island. However, unlike other legendaries in the game, you can't catch it under normal circumstances. You have to use an Action Replay/Gameshark to be able to access Birth Island and catch Deoxys.
In Pokemon Diamond and Pokemon Pearl, there is no possible way to catch a Giritina. You can, however, either catch one in Pokemon Platinum or trade with someone who does have one. In Pokemon Pearl, you catch Palkia at Spear Pillar. In Pokemon Diamond, you catch Dialga at Spear Pillar.