NK had several nuclear tests, and they were not that massive. A nuke made by a member of the nuclear club would have produced far more energy; it's also very unlikely that NK can keep buying or stealing as many nukes as they need. Buying is right out, considering that NK is poor and has nothing to offer to developed countries.
We know that NK has domestic nuclear industry, and we know that have some hackers reworking old rocket designs from all over the world, trying to put something together. By all indications NK's rockets and nukes are made there; perhaps there was help from Pakistan or Iran.
NK tested their nukes which doesn’t eliminate they might have purchased something from Russia.
Russia is working with them.