They are in the crust of the earth like coal and oil, so for earth's short term energy equilibrium they are like coal and oil. They are large heavy atoms that originate when smaller atoms get squashed together so hard they stick.
Well, to some extent anyway. Uranium 235 is the only fissionable isotope found in significant quantities in nature. Even so, naturally occuring Uranium contains such a small ratio of u-235 to U238 that it is necessary to enrich the Uranium to create the fissionable material to create a usable core.
Plutonium-239, another common fuel, is found in such minute quantities in nature as to be worthless. Instead, fuel cores that use Pu-239 are created from U-238, the most abundant isotope of Uranium found in nature.
Uranium cores, even after enrichment, contain more U-238 than U-235 (I believe commercial reactors use about 20% U-235, Naval reactors about twice that level). Some of the U-238 absorbs Neutrons during the reactors life, to form Pu-239. This is one of the primary recyclables that can be obtained from expended reactor cores.
Theoretically, it is possible to create a reactor than can produce more fuel, in the form of Pu-239, than it uses in the form of U-235. However, attempts to build a commercially viable breeder reactor have thus far failed. We do have 'breeder' reactors operating in the world, but so far their efficiency has fallen below theoretical capacity.
V/R
Shapley