You'd better try SNCF by phone (or an SNCF ticket office):

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

On https://www.oui.sncf/ you see what accommodation is still available but you can't book Interrail supplements there.

Hello!

I am about to book tickets for TGV Toulouse-Paris and Paris-La Roche-sur-Yon on the Eurail website, it says “Seat availability unknown” but it seems I can do the purchase anyway (I’m right before payment). Should I? Will I have seats??

Or should I call like you said the SNCF by phone?

Thanks!

“Seat availability unknown” means there are seats available but the booking module doesn't know how many.

Ok!

I’ve booked them then, and I’ve just received the tickets, with seats references!

Thanks again!

Hélène