Where to Obtain: Blacksmith level 6.
The Total Price: 39,000 Gold for 10x Light Saber Z, 750 crystals, 200,000 Gold to combine the items into a Plasma Sword, plus the taken time to get 10x Fire crystals (You can buy these from the market, but its not cheap and not worth it, just grind them).
At the moment you can not get "Keris" from any missions or special rewards which makes it impossible to get this item without using Crystals. As you can see above its gonna be cheaper just to buy it straight away with 500 Crystals rather then spending 750 on multiple Keris. (Version 1.37)