It has a chance to drop from the eggs themselves, it's pretty likely you'll find one before you have the 500 eggs to buy one.
And if you don't it's a week long holiday, plenty of time.
Unfortunately I did not have such luck. I farmed the entire 500 chocolates, in a few hours no less, and was able to get the mount. By Wednesday the hype had died down and I was able to farm up eggs fairly quickly.
Is it expensive? Sure. Is it possible to do? Absolutely.