Where to find shiny Gyarados in Pokemon pearl?

You would need to patiently fish using the Super Rod to get a Shiny Gyarados, for the chances of a Shiny Pokemon appearing is 1/8192. However, the chances of encountering one can be increased by using the Masuda Method. The Masuda Method is where the two parents in the Day Care are from different countries, so the GTS was required back then. The chances of an Egg containing a shiny is now reduced to 1/1638. This method would require training of the new Shiny Magikarp to evolve into a Shiny Gyarados.

An easier way is catching the story-line Shiny Gyarados in Pokemon SoulSilver or HeartGold and trading it to Pokemon Pearl.