NC boxes come from redeeming gift cards, or by opening a gift box mystery capsule. These capsules are released for one week every month for 200 NC ($2), and will give you between one and five boxes at random. The gift cards will give you between two and eight boxes, depending on the value of the card -- the higher the value, the more boxes you receive
Rarely, TNT will release a box in the mall for free, for people to claim. They sometimes do this with NC events as well, but you have to pay to access the event (often around Christmas and Easter).