Plutonium is the element with the highest atomic number to occur in nature. Neutrons are in abundance in supernova, so heavy elements like plutonium can be created in copious quantities. Trace quantities arise in natural uranium-238 deposits when uranium-238 captures neutrons emitted by decay of other uranium-238 atoms. Plutonium is separated from uranium and other transuranium elements by subjecting an aqueous solution in dilute nitric acid of the elements to be separated to electrolytic extraction by counter-current or cross-current ion migration in the presence of a complexing agent, preferably acetic acid or a mixture of hydrofluoric acid with an alkali metal fluoride, until plutonium and the other elements present accumulate in separate portions of the solution which are thereafter separately withdrawn.[1]

[1] This is all copypasta from Wikipedia and firsts of Google search results.