Fares for trains in France for the iDTGV are available through May. The prices are fantastic (Marseilles to Paris €19) if you can commit to specific days and times. iDTGV is only on some trains so not all schedules are available.
I usually use the SNCF website. Is iDTGV a reputable seller of tickets? Sorry I am unfamiliar with them
iDTGV is a subsidiary of SNCF running only TGV trains on certain routes. They often have great rate specials (as does SNCF) on certain TGV lines. I found the same €19 rate for that route on the normal SNCF website. The iDTGV tickets can only be purchased online (guess the i means internet).
As I understand it, the iDTGV operation attaches special cars to regular trains, for certain routes only. On some, you can choose a "quiet" car where phones are banned and even conversation is supposed to be muted. Book early in case this service sells out. According to the SNCF website, tickets are on sale through the end of the summer. http://en.voyages-sncf.com/en/idtgv
I was excited to try this as I have several TGV journeys on my itinerary in June next year. For example, from CDG to Lyon on 3 June. There was a 10 € fare! But it was only for a train at 21.03 arr. 23.08. Far too late for me to reach my hotel in Tournus. And again from Lyon to Montpellier on 16 June: a 10 € fare on 5 and 23 June, but no fares shown for any other date in June. So it may work for some if you are very flexible with dates and times, but it seems I'll have to wait until the standard TGV bookings open.