darkrai/arcecus in emulator

is it possible to get darkrai and/or arcecus on the desmume emulator?

Yes, if there are suitable Action Replay codes available you can use them with the Cheat function within the emulator.

is there a site were i can find these codes?

What pokemon version are you using? I'l guess Platinum.
http://www.neoseeker.com/Games/Products/DS/pokemon_platinum/action_replay.html