While certain areas on Earth have deposits or other elements that can be used to create the isotope, the moon’s soil is rich in it as the lunar surface has been collecting it from the Sun's solar winds for approximately four billion years.
Helium-3 is an isotope of helium, one neutron and two protons in the nucleus instead of the usual two and two found in ordinary helium-4.
Regolith, a.k.a. lunar surface material, has about 28 ppm (parts per million) of helium-4, and only .01 ppm of helium-3. It therefore takes a hundred million tons of regolith to recover one ton of helium-3.
Tritium, which can be found on Earth and has a halflife of twelve years, decays into helium-3, which can be trapped and concentrated for industrial use.
Other potential sources are the atmospheres of the outer gas giants Uranus and Neptune, but man has yet to reach that far out into space.
Modern fusion reactors require helium-3 for the deuterium-helium-3 reaction. It is a process that is less energetic than deuterium-tritium, and needs a bigger power plant, but it is cheaper, simpler, and safer to operate. It also doesn’t pollute the air or water and produces very low-level radioactive waste without using radioactive fuel.
Over 90% of Earth's electrical power now depends on helium-3.
Competition for this isotope is fierce, and was one of the major contributing factors that led to the War.
Mining for the isotope is governed by the Helium-3 mining consortia, whose largest corporation is undoubtedly Melange Mining.
Helium-3 is shipped in large metallic canisters via the Beanstalk.