Wild Chansey have a 5% chance of holding Lucky Egg. You can find Chansey on Route 13, 14, and 15, plus the Safari Zone with objects. Normally, Chansey has a 1% chance of appearing, but this increases to 40% during swarms. Swarms are chosen at random each day, so you can use the DS clock to cycle the swarm until you get Chansey.
Also, a Pokemon with the Pickup ability from level 41-60 has a 1% chance of picking up a Lucky Egg.