Can't tell you because its cheating ? Dude your probably like some little baby or something. You had to have played it at event or can't get it
How Do You Get Oak's Letter In Pokemon Pearl Without Cheats?
You get it from an event
You can't you need cheats only way without cheats is the mystery gift.
No that won't work
Can't tell you because its cheating
There is no possible way to get it without action replay. The nintendo event for it has ended and it is only obtainable through action replay codes. I don't really approve of using action replay, but if you must; use a code you find through google.
You can always tweak your game visit youtube and type in how to get shamyin with out Action replay. But tweaking is really Really hard
Receive the mystery gift ad then go to the building that is locked in canalave city.
I'm sorry my friend, but it's impossible to obtain 'Oak's Letter' without cheats, the events were only in the US and Japan, but if you would like, I could send you a Shaymin from my Pokesav, trade me a Murkrow for it, I'm looking for a Murkrow, thanks.