NX clothing is, in fact, an equipable clothing. Aside from Tot which gives you temporary nx clothing covers, there are no other sources that I can recall at the moment that gives free clothing, aside from occasional events Nexon decides to hold.
On another note: There's Fusion Anvils in the Cash Shop, which you can anvil a certain appearance of an equipment onto another piece of equipment. You have the appearance of one equipment while keeping the stats of the other.
Since NX is just equipment itself (and thus this question makes no sense otherwise), there was an event several months ago during the summer that did give those out freely. Otherwise you either have to wait for another event like that one, buy permanent NX in the FM or just buy it yourself with your own NX.