All plutonium originates in nuclear reactors and is produced by the capture of extra neutrons by uranium-238 to form U-239, which then undergoes a series of decays to form Pu-239:
U-238 + n -> U-239 -> Np-239 -> Pu-239
Some of this plutonium gets consumed by fission before it is removed from the reactor, and some of it gets transmuted to heavier isotopes of plutonium by capturing more neutrons:
Pu-239 + n -> Pu-240
You don't put it in a collander and sift it dude!!