Online it is only possible through Interrail/Eurail. However, there is a bug that makes so that Interrail/Eurail only can sell 10 EUR pass holder seats and not the 20 EUR pass holder seats. 

One way to buy a 20 EUR reservation is by calling SNCF.

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English, no booking fees, reservations are sent via e-mail

For domestic French reservations, you can also call NS. Also no booking fees, they do accept VISA  (+31 30 2300023)

If you live in France, go to a train station and use the ticket machine or go to the ticket office. That's the easiest way. You can also call SNCF.

A machine will still spew out a ticket-not the old cardboard anymore, but a long slip with QR code.

ONline/via mail: call SNCF-or perhaps also if asked from your national society at its gares

Thanks, both of you for responding so fast!