It's possible to book this ticket at any other station in France. You may also use a long distance ticket machine to buy it (it's possible to change the station of departure).
If you’d like to book a regular ticket for the train and bus combo to get there, that’s possible online at SNCF Connect : Réservez vos billets SNCF, bus et covoiturage (OUI.sncf) (sncf-connect.com) for 29 €.
You did ask for a ticket, but just in case you did ask a wrong question and do hold a rail pass: With a rail pass, you may buy a reservation for the train part of the journey for 1,70 € at Train tickets in Europe - Search & Book on Rail Europe (or at the station / ticket vending machine). Then you’d need to buy train reservation and bus ticket separately to get the correct price.