At some point, if you’re going to try to collect them all, you gotta start making concessions. Plutonium doesn’t really occur that much in nature - if you’re getting it, it was made in a lab and most isotopes have the stuff that governments get rightly concerned about people having.
Even the relatively harmless 238 that is the reason way the Voyagers are still voyaging is something that the government (by which I mean literally all of them) gets very strict on controlling. (‘‘Twas an incident where some of that ended up on the bottom of the ocean floor funnily enough)
Maybe he could have done a pacemaker, or a uranium rock with bits in it?
I am curious about what his collection looked like, if he got to the “committing forever jail crimes.” Probably would shit myself with jealousy - I had a hard veto on showing my high schoolers what sodium does in water :(