C/D/N ISOTOPES: Quite a large listing of deuterated compounds and Carbon-13 and Nitrogen-15 labelled compounds. In English or German or French.
Cambridge Isotope Laboratories: Manufacturer of stable isotope labeled compounds and some fine deuterated solvents too.
Isotec: Makes some of the most unique and novel labeled compounds
Medical Isotopes, Inc.: Nearly a complete line of enriched biochemicals. Nice selection of fatty acids, enriched with C13.
NMR Shift Reagents: Supplier of NMR shift reagents.
Silantes GmbH: Stable isotope labeled biopolymers (H2, C13, N15)