Every molecule of pure "heavy water" contains two atoms of deuterium, which is hydrogen of the double-weight form identified in 1931 by Columbia's Harold Clayton Urey. Deuterium is not rare in nature. It is present in ordinary water to the extent of one part in 4,500.* Thus when suitable methods for separating it were worked out, high concentrations of heavy water became common. Nowadays one or two chemical manufacturers list and sell heavy water 99.5% pure.
The case of tritium, triple-weight hydrogen, is different. Its discovery was foreshadowed by the somewhat dubious magneto-optic...