I'm working with doxycycline and related substances (6-epi-doxycycline and metacycline)
I would like to know if there are commercial available standars of related substances that aren't USP or EP standars.

Thank you for your help

You may want to try calling Sigma or Aldrich. From what I hear, they have may compounds that are not necessarily listed in their print catalog.

A general word of caution! If you are doing an assay such as drug substance / product, or other work for submission to a regulatory agency, you must use a USP reference standard (I am not sure about requirements in other countries) or a well characterized house standard which comes with an audit trail of all supporting analysis and documentation. Compounds purchased from places such as Sigma or Aldrich, are not recognized as well characterized reference standards.