At stark mountain
To the immediate North and East of Jubilife at any time of day
Water Pokemon with water attacks
Grass Pokemon with Grass attacks
I am afraid you cant get it at the moment there is only two ways to get the oaks letter along with the azure flute and the membership pass is to go to an event that is giving them away. the other way is to "hack" your game using a cheat device like the action replay however using these devices can seriously ruin your game and sometimes delete your game altogether.
at one time during the year there will be an event going on you go to a gamestop and it has instructions to get oaks letter but you have to have mystery gift.....you click mystery gift then it will say received oaks lette then you go to the nearnest pokemart and a guy will give it to you.....or you can just use gameshark or action replay!! hope this HELPED!!!!