What do you mean not found? It's literally in their constitution :

Article 19. The Democratic People’s Republic of Korea relies on socialist relations of production and on the foundation of an independent national economy.

Article 20. In the Democratic People’s Republic of Korea the means of production are owned by the State and social, cooperative organizations.

Article 21. The property of the State belongs to all the people. There is no limit to the property which the State can own. All natural resources, railways, air transport service, post and telecommunications establishments, as well as major factories and enterprises, ports and banks of the country are owned solely by the State. The State shall protect and develop on a preferential basis State property which plays the leading role in the economic development of the country.

Article 22. The property of social, cooperative organizations is collectively owned by the working people involved in the organizations concerned. Land, farm machinery and ships, as well as small and medium-sized factories and enterprises may be owned by social, cooperative organizations. The State shall protect the property of social, cooperative organizations.