How do I get Oak's Letter in Diamond, Pearl, and Platinum?

Question

EMERGENCY! Can't proceed otherwise.

Comments

wrong tags man.

Answer 1

Oak's Letter is one of the Key Items available through the DNS Exploit, a process that takes advantage of fan-hosted servers to access old Mystery Gifts. It can be complex to set up; refer to our detailed guide for help. A Shaymin caught after receiving Oak's Letter through DNS Exploit is legitimate and will pass hack checks.

Answer 2

Oak's Letter is, in fact, a event item, and thus not needed for anything but catching Shaymin. The event is long over now, so you cannot get it whatsoever, unless you hack it in.
Source: http://bulbapedia.bulbagarden.net/wiki/Oaks_Letter