We are calling the REST version of CreatePassengerNameRecordRQ in Sabre's test environment with no issues. However, there does not seem to be a REST version of GetReservation.
Is this true? Why would only some of the APIs be available in REST?
There is an early version of a new getBooking API that Sabre is in the process of development. Based on what attributes/elements of the Reservation you are need this could meet your needs.
https://developer.sabre.com/docs/rest_apis/trip/orders/booking_management