Just buying it at the train station is probably easiest and cheaper than buying from a travel agency website. However, you probably won’t be able to take advantage of any of the advance purchase special fares you see on the national rail company websites. You should be able to easily buy it at any major train station in Europe. You can buy from the French national rail company website — although not all of the pages on the site are in English. Just make sure you DON’T use USA as your country or you will be switched to raileurope.com, a travel agency that will sell you the ticket at the inflated global price and possibly also charge shipping and handling. You can also try www.tgv.com — again avoiding getting switched to raileurope.

Have you looked for a cheap flight to Avignon or somewhere near?

If you purchase from the SNCF or TGV site make sure it will let you print your ticket at home. It won’t do you much good to have to pick up a ticket in France if you’re leaving from Barcelona.

Also consider the train routing. You’re looking at at least one change of trains to get to Arles, with some trains requiring two and three changes. The route that requires one change actually changes in Montpelier.