I've discovered several options to book tics from Amsterdam to Rennes FR: capitaintrain, Rail Europe and more. Each has differing fares for same route. Which is best bet? Thanks!
Throw out Rail Europe, they do not have all the routes, only the most expensive ones that they can make money on. Capitainetrain can get you print-at-home tickets for as good a price as any individual national railway site with less credit cards hassles. They have contracts with SNCF, Thalys, Eurostar, and DB.
I was just about to say similar to Sam's answer, although I wouldn't have been so complete.
Go with the Capitaine. They have an app as well as website. The app is very easy.
Use whichever is cheapest. That usually means going direct to the source (SNCF), but you may not be able to use your US credit card on that site. If your schedule has a connection in Paris, allow time to travel by taxi or direct subway from Gare du Nord to Gare Montparnasse. Departures that connect in Lille (e.g., 7 a.m.) are more convenient, since you connect within the same station (walking from international side to French side).
Rick’s France rail travel page has the link and tips for buying “Prems” rate TGV tickets through SNCF (cheap, reserved, nonrefundable, available three or four months ahead of your travel date) to print at home and pay with PayPal. It does not tend to work with US credit cards, unless you have an extra level of security set up.