In nuclear reactors, the fission process is controlled and the energy is harnessed to produce electricity. In nuclear weapons, the fission energy is released all at once to produce a violent explosion. Uranium-235 occurs in nature. For all practical purposes, plutonium-239 does not. It can have a number of chemical forms.
Nuclear weapons use plutonium metal.
Plutonium dioxide is used as a component of some nuclear fuels. Two key facilities are needed to obtain plutonium. First, in a nuclear reactor, uranium-238 absorbs a neutron. This leads to nuclear reactions which convert it to plutonium. The plutonium ends up in the spent nuclear fuel along with unused uranium and highly radioactive fission products.