This article is from the Fusion FAQ, by Robert F. Heeter firstname.lastname@example.org with numerous contributions by others.
(Note: there's more information in the glossary too.)
Hydrogen (p): Ordinary hydrogen is everywhere, especially
Deuterium (D): A heavy isotope of hydrogen (has a neutron in
addition to the proton). Occurs naturally at
1 part in 6000; i.e. for every 6000 ordinary
hydrogen atoms in water, etc., there's one D.
Tritium (T): Tritium is another isotope of hydrogen, with two
neutrons and a proton. T is unstable
(radioactive), and decays into Helium-3 with a
half-life of 12.3 years. (Half the T decays
every 12.3 years.) Because of its short
half-life, tritium is almost never found in
nature (natural T is mostly a consequence
of cosmic-ray bombardment). Supplies have been
manufactured using fission reactors; world
tritium reserves are estimated at a few
kilograms, I believe. Tritium can be made by
exposing deuterium or lithium to neutrons.
Helium-3 (He3): Rare light isotope of helium; two protons and a
neutron. Stable. There's roughly 13 He-3 atoms
per 10 million He-4 atoms. He-3 is relatively
abundant on the surface of the moon; this is
believed to be due to particles streaming onto
the moon from the solar wind. He3 can also be
made from decaying tritium.
Helium-4 (He4): Common isotope of helium. Trace component of the
atmosphere (about 1 part per million?); also
found as a component of "natural gas" in gas
Lithium-6 (Li6): Less common isotope of lithium. 3 protons, 3
neutrons. There are 8 Li-6 atoms for every 100
Li-7 atoms. Widely distributed in minerals and
seawater. Very active chemically.
Lithium-7 (Li7): Common isotope of lithium. 3 protons, 4 neutrons.
See above info on abundance.
Boron (B): Common form is B-11 (80%). B-10 20%.
5 protons, 6 neutrons. Also abundant on earth.
Note: Separating isotopes of light elements by mass is not