Where can you get Deoxys in diamond without an action replay?

You cannot get Deoxys in Pokémon Diamond without an Action Replay or similar cheating device. Deoxys is not a naturally occurring Pokémon in the Sinnoh region, which is the setting of Pokémon Diamond.

Deoxys is a legendary Pokémon introduced in Pokémon Ruby and Sapphire and is only obtainable through special events or by transferring it from a game where it was previously obtained.