If it came from Myanmar, things are going to get interesting over there before long. We can't allow this sort of thing. Even the North Koreans have the sense not to sell plutonium through back-alley deals.
They have the raw materials, the equipment and the expertise. Also Pakistan is right next door.
You wouldn't want the ire of the nuclear powers in the world, if you are Iran.
https://www.sciencedirect.com/science/article/abs/pii/S01491...
Presumably the Yakuza got access to the plutonium by illicit means that were substantially cheaper than producing it themselves, meaning they can sell it for what would otherwise be a loss and still make a nice profit. It's quite reasonable that if Iran wanted to say conduct some nuclear weapons research while appearing to comply with IAEA requirements that buying some plutonium off the black market would be the most economical option.