Sites

Cambridge Isotope Laboratories (Popularity: )
http://www.isotope.com/
Manufacturer of stable isotope labeled compounds and some fine deuterated solvents too.

C/D/N Isotopes (Popularity: )
https://www.cdnisotopes.com/
Quite a large listing of deuterated compounds and Carbon-13 and Nitrogen-15 labelled compounds. In English or German or French.

Silantes GmbH (Popularity: )
http://www.silantes.com/
Stable isotope labeled biopolymers (H2, C13, N15)

Medical Isotopes, Inc. (Popularity: )
http://www.medicalisotopes.com/
Nearly a complete line of enriched biochemicals. Nice selection of fatty acids, enriched with C13.