Developing nuclear weapons isn’t hard. You are mistaken if you think it’s lack of knowhow that is stopping others from doing that.
Last I checked, DPRK weapons were good enough to kill the yanks.
A rock is good enough to kill Ruskies. Buying rocks from the other side of the globe wouldn’t be very inspiring though. You were talking about how the US has to buy missile defence systems from Japan. Russia at the moment is buying artillery shells from North Korea.