That item which is Oak's Letter is only available through the use of a Nintendo event or a cheating device, you can't get it any other way.
He gives it to you as a gift for completing the sinnoh pokedex.
By action replay or events that that come rarely
Unlike in other games you just have to see the Pokemon not catch them but when professor oak gives you the natinal pokedex you have to catch the Pokemon
No.
in Route 224
In Diamond, Pearl, and Platinum, talk to Professor Rowan after you see all the Pokemon in the first Dex you receive. He will give you the National Dex.
In his lab
His lab.
nowhere,but you can get one from professor rowan
150 then talk to Professor Rowen.
after beating the E4 the professor will give it to you as a gift
You can use the special app. professor oak gives you once you beat the elite four. Look up the exact location on a walkthrough website.