Sigma-Aldrich provides isotopically labeled products for Protein Structure Determination, Peptide Synthesis, Proteomics, Metabolic Research, Magnetic Resonance Imaging, NMR, Custom Synthesis, cGMP Capabilities, PET, Agriculture, Gas and Gas Mixes. ISO 9001:2000 certified; continuing commitment to quality.
http://www.sigmaaldrich.com/chemistry/stable-isotopes-isotec.html

Cambridge Isotope Laboratories, Inc. (CIL) is the world's premier producer of stable isotope labeled compounds and the world leader in the field of isotope separations with an annual production capacity of 120 kilograms of Carbon-13 and annual production capacity of 250 kilograms of Oxygen-18.
http://www.isotope.com/cil/index.cfm

Manufacturer of radioactive chemicals as well as OEM producer for Perkin Elmer.
http://arc-inc.com

Since 1975 MB has been engaged in the manufacture of radiochemicals, stable isotope labeled and unlabeled compounds.
http://www.moravek.com