Miracle Egg is a crafting material that Evil Core Lv.42 drops when you kill it. The monster is located in the basement level of Garibaldi Temple. You may also kill Golden Bones at the bottom of the Tower of Evermore to get one. There is one in Dracula's Castle too.
Although it's known to be the rarest material found in Castlevania: Curse of Darkness, there's an easy way to get them very early in the game. In Baljhet Mountains, go to the Save Room located just before reaching the arena where you fight the area boss, the Wyvern. Equip your best Luck raising items (Clown Suit or Prince's Clothes, Rare Ring, etc.) and also summon your best Luck raising Fairy-Type Innocent Devil. Exit the Save Room and three Skeletons should spawn, go into the Save Room and exit it again. Keep exiting and entering the Save Room until Golden Bones appears instead of the three Skeletons, defeat it and he should drop the Miracle Egg. You don't have to fight the Skeletons every time you exit the Save Room. Golden Bones always spawns alone, so as soon as you see more than one spawning enemy, you can enter the Save Room and try again.
Even very early in the game, when your Luck stat should be around 20 and your Fairy-Type Innocent Devil only grants Luck +1, it doesn't take much time until you get a Miracle Egg.