Polonium, Co60 seem to be only options and Polonium is not easy to find.
Iridium 192 would work but is highly controlled and extremely dangerous to its handlers.
Likely polonium. He wanted something fast acting when ingested. Co-60 not all that more lethal when ingested than just being in the proximity, also if you have enough Co-60 to be “lethal” in a short period of exposure you need a semi to transport the shielding cask. Otherwise you die of the radiation exposure before you can position the Cobalt near your target. If I had to guess any web site selling polonium is a sting site run by the FBI.