One could buy Polonium as a private citizen since it was extensively used as an anti-static agent for car painting and photographic development. The dose allegedly
given to Litvinenko was not in the kilogram range but microgram range, easily scavenged from commercial sources.
I remember that it was mentioned at the time that Polonium is not just highly radioactive... ie if you killed someone using it and they were cremated the level of radiation from the polonium would still be present and show what killed the person without any autopsy being given... it is also rather expensive.... hundreds of thousands of dollars per gramme... but not one single reporter even speculated that this might have been a case of Polonium smuggling gone wrong... where the smuggler accidentally came into contact with the product and essentially killed themselves... which actually makes it rather funny.