Where do you find Atlantean Map Piece in Facebook Castle Age

The Map pieces are Epic drops from battling the following color Serpents:

Ancient - piece (1/4)
Amethyst - piece (2/4)
Sapphire - piece (3/4)
Emerald - piece (4/4)

If you keep battling the Emerald serpent, you will keep receiving piece 4 of 4.