How to get a SIREN number (French charity organization)?

A French charity organization only need a SIREN number if it has employee(s), if it has to pay VAT / IS or if it get subsidies.

So you can request a SIREN number from the INSEE if you plan to ask for subsidies. In that case, you'll have to contact the INSEE regional direction, status copies + JO publication.

See http://www.insee.fr/fr/publications-et-services/default.asp?page=services/faq/q42_2demarches.htm