Where do you buy the Retired Egg in Adopt Me?

Excuse me, could you please clarify where exactly one can purchase the Retired Egg in the popular game Adopt Me? I've been hearing a lot about it and am eager to add it to my collection, but I'm having trouble locating the specific vendor or in-game location where it's available. Is it through a specific event, a certain in-game store, or perhaps a trade with another player? Any information you can provide would be greatly appreciated.