There is no Togepi Egg in Nacrene. I think you're mixed up with the Happiny Egg, which can be found from a Breeder in the gate than connects Route 3 to Nacrene City.
If you are a male however, you may find Yancy in Nimbasa City who will have a chance of trading you one in return for any Pokemon, but only once you have called her 50 times.