There are a few ways that you could get a shiny Deoxys in Pokemon Platinum. One way would be to get someone who has hacked one to trade.
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If You Want A Deoxys In Pokemon Platinum ,Migrate,Trade,Or Use The Action Replay Pokemon Modifier.
...shiny Pokemon are always shiny... ._.
If you have Deoxys in the Pokedex, you can find it in the GTS if anyone's trading one.
On Action Replay press disable Shiny Pokemon.
The final deoxys event for Pokemon platinum ended. It was a download mission in Pokemon Ranger Guardian Signs. You had to do it with 4 people. If you have the mission downloaded then you can get a deoxys everytime you beat the mission. Also to be able to play the mission you have to unlock the past missions in POkemon Ranger Guardian Signs.