You mean United Nuclear, which brokers[1] radio isotopes to the general public? https://unitednuclear.com/index.php?main_page=index&cPath=2_... I don't see any Plutonium for sale, but that's not as interesting or useful as Cesium-137 or Polonium-210. They also sell Uranium ore, and do so directly, not brokered, so they have to be careful to vent Radon from their storage room.
[1] IIRC, the Polonium-210 I received was sent directly from Oak Ridge National Laboratory.
EDIT: The sample was actually sent by Spectrum Techniques (https://www.spectrumtechniques.com/). They must be ORNL adjacent or something.