A very reliable site that does seem to compile all of the train options is Trainline.eu. They are the old Captain Train, I believe, and have expanded beyond their original scope. Their payment system is easy, and you can find clear information about how to retrieve your tickets on arrival via the machines or the ticket windows. I can recommend the iDTGV cars - the Zen option is pleasant (you could've heard a pin drop) and if you have the time, an overnight in Montparnasse is a very nice break to rest before moving on (creperies, hammam, Tour Montparnasse and stores for last-minute items, patisseries, museums, a hike around Paris.....). Unless you take the train right out of CDG as others have said above, more trains leave out of Gare Montparnasse (their station for most of the Southbound trains).
Bourricot Express is a very good service also, and Caroline can be counted on for reliable information about going up into the Pyrenees via the Napoleon. She is a mountain guide.