you have to trade/migrate it from a different game that has a deoxys or buy a action replay
only with action replay!!! there is no way without action replay
You use an action replay.
You can't without an action replay
Mystery Gift.
You can't.
you use an action replay
where to find a azure flute without action replay and cheating in pokemon platinum
You can't get the Secret Key in Pokemon Platinum without Action Replay, unfortunately. You can get Action Replay from most major websites.
You can't.
Sadly, no.
You don't
You either have to migrate Pokemon or receive them in trades