Go to the Imp Prison on Dath (its a POI) and walk into the building which is in the middle of the prision,. Go to the room to your left and speek to the guy there, he will give you a mission. Then go to the room that is to the right of the entrance and get his mission right after. Go to the waypoint he gives you and kill the nightsister. Check your inventory, and there will be a Nightsister crystal there. You can do this as many times as you like.
Be sure to talk to the imp on your left first, or the trick will never work again.
You can be rebel or imperial for this mission even though you are doing this for an imperial. You will not gain/lose faction points either.
Note: these are ONLY for decoration, and you will probably only make a 75k profit off of each one of these, but hey eth! you can do this mission in under 5 minutes.