Here is how you get your hands on plutonium legally.

In both cases it may still be illegal to smuggle into the country and therefor you will need a local supplier after obtaining the proper license. The easiest and least harmful would likely be an ore containing trace amounts of a safer isotope. For higher purity you would need a refined product likely only available at government facilities and contractors.

Ever since the Nuclear Boyscout incident it’s been a lot harder to obtain radioactive elements without tons of paperwork and red tape, and for good reasons.

In Australia:

Permit to Possess Nuclear Material LINK HERE

in the USA:

Get a certificate to use depleted uranium under a general license LINK HERE