1. The "white color" shadow gear you see from some people, like "immune/hard", come from kafra shop boxes. They come randomly from the box. Right click the box in game to see what you could get from the box.
shadow shield box 200kp
shadow stat box 200kp
shadow armor weapon box 200kp
physical shadow package 1800kp
magical shadow package 1800kp
shadow box 200kp
You can also buy these gears from other players through vends/trades.
2. The "black color" shadow gear you see from some people, like the Shadow "Armor/Head/Footgear/Weapon" came from a free in-game event, where you kill these black color giant thief bugs, that drop the pieces randomly. The event ran two times so far.
August 2014 to October 2014
January 2015 to February 2015
so they might hold the event again sometime in the future. But for now there is no way to get these gears other than through the event since they are account bound.