The term "nuclear" in the chemistry/physics sense refers to chemical reactions involving the nucleus of cells. Most chemistry and chemical reactions involve the electrons of atoms, but nuclear chemistry involves the fusing together or breaking apart of atomic nuclei. Nuclear chemistry is used to make weapons of mass destruction and also to generate electricity. The most common type of nuclear chemistry is fission whereby a large atomic nucleus is broken apart to release energy. By far the two most common elements for this type of use are uranium and plutonium.
As for the second part of your question, nuclear materials are not readily sold anywhere. You must mine and process your own nuclear material for use. Uranium is actually not too rare an element found naturally in the Earth's crust. The problem is that only a particular isotope of uranium (U-235) is useful for nuclear fuel, and this isotope is only present as <1% of natural uranium. So it must be separated from the other 99% of uranium which is a difficult and expensive process.