Plutonium was generated in reactors in the US but those were all shut down by the late 80s. It's not an easy thing to just mine or make more of there were whole reactors basically dedicated to just what we have left today. In fact we're kind of running out of the stuff for scientific missions, NASA only has enough for one RTG left according to reports.

https://fas.org/sgp/othergov/doe/pu50yc.html

https://mjosefweber.medium.com/nasa-needs-plutonium-ea3bbaad...