The first A-bomb dropped on Hiroshima consisted of two lumps of uranium, each below critical mass, that were brought together by high explosives. The bomb that destroyed Nagasaki held a single lump of plutonium. It was surrounded by high explosives, which compress the plutonium uniformly, creating the critical mass. Their best option would be to steal the stuff, which is why many worry about chaotic conditions in the former Soviet Union. While nuclear weapons are closely guarded, there are other potential sources, such as laboratories, not so well maintained.
Russian authorities deny that their country leaks weapons materials from all sides.
They claim that all of the cases of nuclear smuggling were schemes by the German Federal Intelligence Service intended to discredit Russia's nuclear power industry. But he agrees that security in power reactors has been found wanting in some cases. For the amateur it is easier to make a bomb out of uranium, but it takes as much as 30 kg. Plutonium bombs are harder to make work, but require less material. The International Atomic Energy Agency claims 8 kg is the minimum, but some experts think this is deliberate disinformation designed to deter would-be proliferators, and that as little as 3 kg will suffice.