Where do you go to catch Shaymin in Pokemon Diamond?

Getting Shaymin

The only ways to get shaymin are: 1. use the walk through walls code and walk all the way north on route 224 by the white rock. 2. use the surf glitch in the Japanese game. 3. wait for the Nintendo event to happen in America.
You get the Oak's Letter via Nintendo wifi event (or cheat=)
I Assume You Mean Shaymin, and you can only catch it via Nintendo wifi event or trade, im not sure when the next shaymin event is however.