I would not try to often, people gotten permabanned by their anti fraud system for trying once to often to purchase ncoin. I would buy it from amazon which won't screw you over with their anti fraud system http://prntscr.com/afaf9n
http://www.amazon.com/NCsoft-NCoin-1600-Online-Game/dp/B01ACNVSZ0

Here's the link for Ncoins on Amazon

Still there? http://www.amazon.com/NCsoft-NCoin-1600-Online-Game/dp/B01ACNVSZ0
Whatever you do, I would not buy from ncsoft directly.
https://www.reddit.com/r/bladeandsoul/comments/4g0nmz/ncoins_for_trove_keys_got_me_banned/