Why? The level of technology required?
If you're talking about them buying a functional nuclear device, it is (very slightly) possible but the weapon would have to be maintained. Just having it sit around for a while would greatly reduce the chances it would function as designed. That is why the mythical "suitcase nukes" are not a threat. They have long since passed their shelf life. (True "suitcase nukes" don't exist but the smallest nuclear weapons require very regular maintenance in order to function.)