what is the official site to buy TGV tickets
This should get you there:
http://www.sncf.com/en/passengers
Lots of folks prefer this, most find the interface and credit card functions to be better, same pricing and availability as SNCF:
www.capitainetrain.com
An old trick, when using the SNCF site, if you are in North America, is to specify a location somewhere else for ticket delivery. It avoids an automatic bump to the RailEurope site which has an SNCF tie-in but is a poorer shopping option. I specify Antarctica, not even a country and certainly lacking both permanent residents and trains. Works fine: I print the tickets at home, if available, or pick them up at an SNCF location in France.
rome2rio is another site that helps lay out transport options
I'm a big fan of Capitaine Train. Their website is streamlined, user friendly, in English and their customer service is great. Have purchased tickets from them several times. Check them out. www.capitainetrain.com They also have an app for your phone.
Lorri