Until about November, you won't be able to look up exact timings / services, as the timetable changes in December and they won't publish it until about November. Tickets will then be sold about 3 months out, and you'll want to book any TGV legs at 90 days to secure the cheapest prices. (High speed trains in France are priced much like airline tickets, complete with budget-like options from idTGV and Ouigo!)

Unless you do something odd with slow trains, your easiest is to head from Chartres to Paris, change stations, then take a TGV down to Avignon. Well, either that, or train to Paris, RER to Marne la Vallee, then Ouigo down.

There's a handful of direct TGVs from Tours to Avignon, but getting there from Chartres isn't that easy. You could take a local train to Le Mans, then TGV to Nimes, then local train on, but via Paris looks easier than that to me!