Animal Crossing New Leaf

Obtained: To Obtain the Golden Shovel, You need to Unlock the T&T Emporium, If you Don't have this, unlock and come back.

Ok, so when you have the T&T Emporium, Buy 50 Fertilizer  from Leif, and on the 50th time he will give you a "Happy Bonus", and it will be the Golden Shovel.

Animal Crossing City Folk

Bury a shovel for twenty four hours (Time Traveled can be used) and dig it up again and it will be golden.

Animal Crossing Wild World

Same as City Folk.