USP's official Reference Standards are highly characterized specimens of drug substances, excipients, impurities, degradation products, dietary supplements, compendial reagents and performance calibrators. They are required for use in conducting official USP–NF tests and assays.
http://www.usp.org

LGC Promochem is Europe’s most comprehensive source of reference materials.
http://www.lgcpromochem.com/home/home_en.aspx