How do you get Oak's letter in Pokemon Diamond?

Question

I need it because is an EMEGENCY

Answer 1

It is an event item. you can use it to get shaymin if I remember correctly, but the event is already over, so you can't get it. :(

http://www.serebii.net/itemdex/oaksletter.shtml

Answer 2

Oaks Letter was a speical event to catch shaymin. But it was only available from Sept 28 --- Nov 8 2009

Sorry the event is over!!

Answer 3

I think the more interesting question is how did you not see the duplicate question ;)
It would have come up when you asked yours:
http://pokemondb.net/pokebase/234/where-you-get-oaks-letter-in-pokemon-diamond