I've been trying to book one-way tickets from Paris to Giverny (Vernon station) and have tried both www.voyages-sncf.com in French and www.tgv-europe.com using the UK as my home country. On the French site I can go into the timetable/schedule and see all of the trips and times for that day, but when I go to book that route at any particular time it tells me that route does not exist. On the UK site, it doesn't even list Vernon as a station so then I used the map planner selecting Paris as my point of departure and Mantes la Jolie as my point of arrival (it doesn't show Vernon, and on the French site it looks like the majority of the trains go through Mantes la Jolie so I chose that as the closest point) and it gives me an error saying there is no direct route.
Is this a route that can't be booked in advance? And if this is the case, how far in advance should I plan to arrive at the station in Paris to make sure we get the train we want?