Let me start by saying I have no desire or intent to ever even think about buying enriched plutonium or "yellow cake" or anything like that. Do you guys think that if it were somehow available on the DNMs there'd be people who were successful in doing that transaction? I'm sure every case would end in America: World Police saving the day again, but I'm having fun pondering the idea.
It is far more realistic to imagine ten years from now, fringe scientists take to DNMs and sell the DNA of premiere athletes/high profile politicians/brilliant academics/super elite/super models/psychopathic mastermind criminals/highly effective killers (military forces included) in an attempt to recreate humans through in vitro genetics with specific genes from said DNA. Though, it is also entirely possible rare diseases which have no known cure to date may be synthesized in a way that allows them to contaminate people for the purpose of either addressing a need to develop a cure and/or kill/control many in a population. If you want to daydream, think more about the nefarious possibilities of biotechnology.
Anything nuclear is likely to be sniffed and prevented as fast as the NSA can locate any average internet user. Far too much interest in all things potentially powerful to cause devastating radiation.