If you didn't attend the PlayStation E3 event in 2015 or buy the Arkham Collection, now's your chance to get the Earth 2 Dark Knight skin in the PlayStation Store!  I found out from Batman Arkham Videos that you can also use the Earth 2 skin on PC in the Epic Games Store version of Arkham Knight, or install it as a mod on Steam.

HOW TO INSTALL EARTH 2 DARK KNIGHT SKIN ON STEAM; Download this file: http://www.batman-arkham.com/upload/B

Extract the folder "DLC" in the main game's directory. This mod includes the Earth 2 Dark Knight Skin, Zur En Arrh Batman Skin, Anime Batman Skin and Joker Batmobile. 

Batman Arkham Videos Credit:  https://www.youtube.com/watch?v=Og1iNirSn5I&t=42s