The only way to get Oak's Letter as a Mystery Gift was to wait for an event, go to that event, and then download it as a mystery gift. However, the event to get Shaymin in America for Diamond and Pearl has already passed, so it is impossible to get it as a mystery gift now. There has already been a Pokemon Event for Shaymin (what you need Oak's Letter for) for Pokemon Platinum, so that has passed, too.
it's impossible without mystery gift action replay or gameshark
you have to get it in mystery gift or an action replay
You would have to go to an Event where they had Mystery gifts. But the events are past, so you cannot get one without an Action Replay now.
You can't anymore it's IMPOSSIBLE You can only obtain Oak's letter as a mystery gift. This mystery gift was given at selected cinemas for the Pokemon Shaymin Movie. This event has now ended, making it (for the moment) impossible to get Oak's letter without action replay.
You do not need an action replay. Oak's Letter can be obtained from the Wifi Mystery Gift.
you need an action replay
Using an action replay.
=Its not possible but getting aceus is without action replay=
by action replay
you can not besides friends
Mystery Gift.
action replay