Hello MaryCV
On the Bernina Express, any ticket is valid that is either for the route, a day pass, or a travel pass such as the Swiss Travel Pass or Eurail Pass. You can purchase it from any reseller, however, it makes sense to get the ticket from SBB, www.sbb.ch, or through the SBB Mobile app
If want to book a seat for the Bernina Express, this can be done from the Bernina Express website directly (only).
Does that answer your question?
Kind regards,
Roland