This is not the only way. you have to go to stark moutain and capture heatran(after you have worked with Buck you find him outside stark mountain). The best way to do this is complete up to the part where you receive the master ball trade it on to Pokemon pearl catch heatran lvl 70 then go to eterna city head to the south and visit the house on the left. The fly to floaroma town and go to the pokemart and you speak to a dude and receive the letter
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You have to get it from an event, but there hasn't been any event lately for Pokemon diamond/pearl. Most of the events are for platinum now, oaks letter was given out a feew weeks ago for platinum. You can keep a look out though by going to serebii.net
No because it does mater
YOu CAN GET A SHAYMIN BY getting the preffesors oaks letter from an Pokemon event or by using an action replay. for more info go to yourpokemongo.webs.com
you go to route 224 in Pokemon diamond and oak will be there and u can catch shaymin!
You can't get it without using action replay. The code is down below 94000130 fcff0000 B21c4d28 00000000 B0000004 00000000 0000aa3c edb88320 2000aa68 00000009 D2000000 00000000