There's the Lake of Rage in Johto, since you have the Red Gyarados.
Other than that, Shiny hunting is mainly luck based. You can use the Masuda Method to get shinies, but even then it's still very unlikely. If you want a shiny rare pokmeon, you might consider saving and resetting until you luck out and get them. The Pokeradar method is also of use, but again, there's no specific location that gives you a better chance of getting a shiny Pokemon.