Where do you catch Togepi in Pokemon platinum?

Answer 1

In eterna city Cynthia should've gave u an egg. It hatches into a togepi.

i agree but here is more info if ya need it. After defeating Jupiter in Eterna City, Cynthia will give you an Egg that contains a Togepi.

if you would like to catch a wild togepi and not hatch an egg then listen closely.

after you defeat the Pokemon league you can check your Pokedex and it will tell you where you can catch it:
west of the resort area you use surf until you see a big patch of tall grass, you can use the pokeradar to find a togepi there.