Proper reference for the International System of Units

Yes, the BIPM one – the NIST describes itself as ‘United States version of the English text of the eighth edition (2006) of the International Bureau of Weights and Measures publication Le Système International d’ Unités (SI)’.

When I've cited it, it's been as

BIPM. Le Système international d’unités / The International System of Units (‘The SI Brochure’). Bureau international des poids et mesures, eighth edition, 2006. URL http://www.bipm.org/en/si/si_brochure/, ISBN 92-822-2213-6.

Or...

@Book{si-brochure,
  title =        {Le Syst\`eme international d'unit\'es / The
                  International System of Units (`The {SI} Brochure')},
  publisher =    {Bureau international des poids et mesures},
  author =       {{BIPM}},
  howpublished = {Online},
  url =          {http://www.bipm.org/en/si/si_brochure/},
  edition =      {Eighth},
  isbn =         {92-822-2213-6},
  year =         2006
}

Or for after the 2019 redefinition:

BIPM. Le Système international d’unités / The International System of Units (‘The SI Brochure’). Bureau international des poids et mesures, ninth edition, 2019. URL http://www.bipm.org/en/si/si_brochure/, ISBN 978-92-822-2272-0.

@Book{si-brochure,
  title =        {Le Syst\`eme international d'unit\'es / The
                  International System of Units (`The {SI} Brochure')},
  publisher =    {Bureau international des poids et mesures},
  author =       {{BIPM}},
  howpublished = {Online},
  url =          {http://www.bipm.org/en/si/si_brochure/},
  edition =      {Ninth},
  isbn =         {978-92-822-2272-0},
  year =         2019
}

The BIPM one, not the NIST one. The PDF file has an ISBN which is 92-822-2213-6