Hello Theeraullee44
The Glacier Express is not operated by SBB but the partner railway company Rhaetian Railways, short: RhB.
Certain agencies do have a fixed contigent of seats on each train which they sell on their own.
For seat availability, please e-mail the RhB rail service which is the Glacier Express customer service. It is well possible that certain groups or agencies will cancel seats at the last minute which are again made available for individual travelers:
E-mail: info@glacierexpress.ch
Phone: +41 81 288 65 65 (from the U.S.: 011 41 81 288 65 65)
Kind regards,
Roland