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.

Thanks for the help!

The pass should also be valid on the connecting bus. The bus supposedly has mandatory reservations and you can always ask for a reservation at an SNCF ticket office, but I doubt they can make one.

Last year, I tried to get a reservation for the bus from Bréauté-Beuzeville to Étretat, at an SNCF ticket office, but that wasn't possible, although SNCF claim it has mandatory reservations. In practice, it was a regional bus that could also be booked as a connection with a specific train, but only for regular tickets. There were no assigned seats. They just called for a 2nd bus when it turned out to be busy. Passes were accepted without issues; the driver seemed to know mobile passes, because he immediately scrolled down to check if his bus was listed.