After you beat team galactic's boss and catch/defeat giratina in the distortion world you will be able to get to sunnyshore city and after you beat the gym leader continue to the elite four.
sunnyshore
you surf
you can get it before you get to sunnyshore city
You need to battle Girtina in the Reverse \World then you will be able to go to Sunnyshore
in sunnyshore
i believe you get it in veilstone or sunnyshore
sunnyshore city
Sunnyshore City.Defeat Volkner first...
In A Grass Patch Close To Sunnyshore City.
at the front of sunnyshore city
on the left of sunnyshore city. a fisherman has feebas
first you you have to catch Giratina