TGV Paris to Barcelona

PSM,

You can always buy your train tickets in France, but if you buy in advance you may get a discounted price.
I usually buy 2nd class train ticket (less expensive) but if you want to buy 1st class, that's a personal preference.

You can buy this type of ticket up to 3 months in advance.

You will want to use this website: http://en.voyages-sncf.com/en

You may want to read this RS article on buying point-point train tickets in France:https://www.ricksteves.com/travel-tips/transportation/trains/france-rail-passes#ticket-tips

EDIT: It might be less expensive to fly from Paris to Barcelona. Check flights here: http://www.skyscanner.com

I think second class is fine. Buy soon for a better price in either class if you are ready to lock in a travel date and time. Cheaper tickets have limited seats available and are not refundable. You could compare Rail Europe and www.capitainetrain.com, both reliable agents. To compare flights, see www.skyscanner.com.

Voyages SNCF does not take some US credit cards and did not accept PayPal for this particular route, in my previous tests.

Vueling air will get you from Paris Orly to Barcelona in 1H35m. Price is 1/3 of train ticket.