And you're absolutely right about differences between cesium and osmium. They couldn't be any more different, which is why it's ludicrous for someone to mention one when they meant the other.
Osmium is a very hard metal that we use in alloys for lots of different things. As with many rare elements, the highest concentrations of it are found in Russia. During the Cold War, the West generally obtained them from the former Soviet Union through shell companies and other means of disguising the original buyer. It was never smuggled per se.
137Cs, is a product of uranium fission and therefore comprises a large proportion of the waste products of commercial nuclear reactors. It has an energetic decay process. Cesium ignites spontaneously in air. The parody site quoted by Vixen concocted a story by which a truck carrying cesium set the car deck on fire and the crew tried to open the bow ramp to push it out. Vixen bought it hook-line-and-sinker and then tried to bluff her way around it.