Where to catch shiny magicarp in Pokemon pearl?

I'm not thinking of an actual specific spot to catch a shiny Magikarp, I'm pretty sure you have like a 1/1000 chance of any Magikarp being shiny, the only Pokemon i can think of, that you actually never can see as a non-Shiny version, is the Red Gyrados you see in Gold, Silver, Crystal, Heart Gold, or Soul Silver. That is located in the Lake of Rage, north of the town with the seventh gym in it...... Mahogany i think.