The other 2 answers are wrong because Cynthia only gives you togepi in platinum due to its not in the sinnoh dex in diamond and pearl.
You cant actually get togepi untill you have beeten the pokemon leuge and got the national dex and pokeradar.
The image is to big so go here
But use your pokeradar on rt 230 and you should have a 22% chance of finding one