The Offertory chant is probably the most neglected proper in the ordinary form of the Mass. The overwhelming habit here is to just sing some random hymn or listen to some piece that the pianist or small choir has worked up. Most Catholics are shocked to learn that there is a specific chant and text assigned to his procession as part of the liturgical structure of the Roman Rite.
The offertory text for the 9th Sunday in Ordinary time is Sperent in te. Here are some options.
The Anglican Use Gradual which is also in print:
The Simple English Propers
The Simple Choral Gradual by Richard Rice:
The Palmer-Burgess Gradual which is also in print.
And finally the normative option, the Gregorian from the Roman Gradual or Gregorian Missal, and an MP3 of the same.