As long as they don't rely on foreign supply for their missiles[1], they can manufacture them forever without having to pay anything to anybody.
[1]: it may sound like a bold hypothesis, but I doubt anyone would ship missile materials to North Korea. China could, of course, but that would be a bit risky diplomatically.
The place is an autocracy. The only way they will run out, is if nobody, and i mean absolutely nobody, wants to trade with them, officially or under the table.
And with NK slaves showing up in Polish shipyards, that is highly unlikely.
why would they need money? Why would they need to trade with anyone? It is not like North Korea is infinitely small and the earth infinitely big. The earth is a small globe, and NK is a region of that globe, one which almost certainly can be an autarky.