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 permit process asks you what amounts you will obtain and who the supplier will be even before permission is issued. 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

EDIT: You WILL have a surprise inspection and tbey WILL confiscate harmful materials if you don't have a license and specific need for them, eg polonium.