Since there are no natural sources of californium, the element must be prepared by nuclear synthesis; the known isotopes may be obtained by charged particle bombardments, the heavier ones by neutron irradiation. It is also possible to find compounds among the byproducts of thermonuclear explosions.

No matter the process used to obtain californium, this element represents always a small fraction of the reaction byproducts. Usually fractionizing techniques are required to separate the element.