Where to catch Magikarp

In Paldea (Scarlet & Violet)

Dalizapa Passage — Walk
East Paldean Sea — Walk
East Province (Area Three) — Walk
East Province (Area Two) — Walk
Glaseado Mountain — Walk
North Paldean Sea — Walk
North Province (Area One) — Walk
Poco Path — Walk
South Paldean Sea — Walk
South Province (Area Five) — Walk
South Province (Area Four) — Walk
South Province (Area One) — Walk
South Province (Area Six) — Walk
South Province (Area Three) — Walk
South Province (Area Two) — Walk
Tagtree Thicket — Walk
West Paldean Sea — Walk
West Province (Area Three) — Walk
West Province (Area Two) — Walk

In Kitakami (Scarlet & Violet – The Teal Mask)

Chilling Waterhead — Walk
Fellhorn Gorge — Walk
Infernal Pass — Walk
Kitakami Wilds — Walk
Mossfell Confluence — Walk
Oni Mountain — Walk
Oni's Maw — Walk
Reveler's Road — Walk

In Hoenn (Ruby, Sapphire & Emerald)

Abandoned Ship — Good Rod, Old Rod
Dewford Town — Good Rod, Old Rod
Ever Grande City — Good Rod, Old Rod, Surf
Lilycove City — Good Rod, Old Rod
Meteor Falls — Good Rod, Old Rod
Mossdeep City — Good Rod, Old Rod
Pacifidlog Town — Good Rod, Old Rod
Petalburg City — Good Rod, Old Rod
Route 102 — Good Rod, Old Rod
Route 103 — Good Rod, Old Rod
Route 104 — Good Rod, Old Rod, Super Rod
Route 105 — Good Rod, Old Rod
Route 106 — Good Rod, Old Rod
Route 107 — Good Rod, Old Rod
Route 108 — Good Rod, Old Rod
Route 109 — Good Rod, Old Rod
Route 110 — Good Rod, Old Rod
Route 111 — Good Rod, Old Rod
Route 114 — Good Rod, Old Rod
Route 115 — Good Rod, Old Rod
Route 117 — Good Rod, Old Rod
Route 118 — Good Rod, Old Rod
Route 119 — Good Rod, Old Rod
Route 120 — Good Rod, Old Rod
Route 121 — Good Rod, Old Rod
Route 122 — Good Rod, Old Rod
Route 123 — Good Rod, Old Rod
Route 124 — Good Rod, Old Rod
Route 125 — Good Rod, Old Rod
Route 126 — Good Rod, Old Rod
Route 127 — Good Rod, Old Rod
Route 128 — Good Rod, Old Rod
Route 129 — Good Rod, Old Rod
Route 130 — Good Rod, Old Rod
Route 131 — Good Rod, Old Rod
Route 132 — Good Rod, Old Rod
Route 133 — Good Rod, Old Rod
Route 134 — Good Rod, Old Rod
Safari Zone — Good Rod
Seafloor Cavern — Good Rod, Old Rod
Shoal Cave — Good Rod
Slateport City — Good Rod, Old Rod
Sootopolis City — Good Rod, Old Rod, Super Rod, Surf

In Johto & Kanto (Gold, Silver & Crystal)

Blackthorn City — Good Rod, Old Rod, Super Rod, Surf
Cerulean City — Good Rod, Old Rod, Super Rod
Cherrygrove City — Good Rod, Old Rod, Surf
Cianwood City — Good Rod, Old Rod
Cinnabar Island — Good Rod, Old Rod
Dark Cave — Surf
Dragon's Den — Good Rod, Old Rod, Super Rod, Surf
Ecruteak City — Good Rod, Old Rod, Super Rod
Fuchsia City — Good Rod, Old Rod, Super Rod, Surf
Ilex Forest — Good Rod, Old Rod, Super Rod
Lake of Rage — Good Rod, Old Rod, Super Rod, Surf
Mt. Mortar — Old Rod
Mt. Silver — Old Rod
Mt. Silver Cave — Old Rod
New Bark Town — Good Rod, Old Rod
Olivine City — Good Rod, Old Rod
Pallet Town — Good Rod, Old Rod
Route 10 — Good Rod, Old Rod, Super Rod
Route 12 — Good Rod, Old Rod, Super Rod
Route 13 — Good Rod, Old Rod, Super Rod
Route 22 — Good Rod, Old Rod, Super Rod
Route 24 — Good Rod, Old Rod, Super Rod
Route 25 — Good Rod, Old Rod, Super Rod
Route 26 — Good Rod, Old Rod
Route 27 — Good Rod, Old Rod
Route 28 — Good Rod, Old Rod, Super Rod
Route 30 — Good Rod, Old Rod, Super Rod
Route 31 — Good Rod, Old Rod, Super Rod
Route 32 — Good Rod, Old Rod, Super Rod
Route 34 — Good Rod, Old Rod
Route 35 — Good Rod, Old Rod, Super Rod
Route 42 — Good Rod, Old Rod, Super Rod
Route 43 — Good Rod, Old Rod, Super Rod, Surf
Route 44 — Good Rod, Old Rod, Super Rod
Route 45 — Good Rod, Old Rod, Super Rod, Surf
Route 6 — Good Rod, Old Rod, Super Rod
Route 9 — Good Rod, Old Rod, Super Rod
Ruins of Alph — Old Rod
Sea Route 19 — Good Rod, Old Rod
Sea Route 20 — Good Rod, Old Rod
Sea Route 21 — Good Rod, Old Rod
Sea Route 40 — Good Rod, Old Rod
Sea Route 41 — Good Rod, Old Rod
Slowpoke Well — Old Rod
Tohjo Falls — Good Rod, Old Rod, Super Rod
Union Cave — Old Rod
Vermilion City — Good Rod, Old Rod
Violet City — Good Rod, Old Rod, Super Rod
Viridian City — Old Rod
Whirl Islands — Good Rod

In Kanto (Red, Blue & Yellow)

Celadon City — Old Rod
Cerulean Cave — Old Rod
Cerulean City — Old Rod
Cinnabar Island — Gift, Old Rod, Super Rod
Fuchsia City — Old Rod, Super Rod
Indigo Plateau — Old Rod
Pallet Town — Old Rod
Route 10 — Old Rod
Route 11 — Old Rod
Route 12 — Old Rod, Super Rod
Route 13 — Old Rod, Super Rod
Route 17 — Old Rod, Super Rod
Route 18 — Old Rod, Super Rod
Route 22 — Old Rod
Route 23 — Old Rod
Route 24 — Old Rod
Route 25 — Old Rod
Route 4 — Gift
Route 6 — Old Rod
Safari Zone — Old Rod, Super Rod
Sea Route 19 — Old Rod
Sea Route 20 — Old Rod
Sea Route 21 — Old Rod
Seafoam Islands — Old Rod
Vermilion City — Old Rod
Viridian City — Old Rod

Magikarp — full Pokédex entry