Unless you have access to a game from another region of the world that would give you access to Masuda Method, the best way to hunt Magikarp would be in one of its guaranteed surfing spots.
Magikarp is a guaranteed surfing encounter in all of these spots:
If I were to pick from those, Route 45 would probably be the best choice, as it has a long, straight river that is easy to navigate. You could also go to the Dragon's Den, where there's 90% Magikarp and 10% Dratini, which imo would be a pretty good hunt.
Keep in mind this is 1/8192 odds, so this may take a very long time. Good luck!