I personally came across this animal a quarter of a century ago.
back then, you could get a free (basic) S/MIME certificate from: www.thawte.com
today, three examples of providers:
- www.globalsign.com/fr/securisation-email
- www.digicert.com/fr/tls-ssl/secure-email-smime-certificates
- www.swisssign.com/fr/fachartikel/sicherheitsrisiko-e-mail.html