Sounds like you had really bad luck at your particular Bunnings. They are normally better than that.
Part of the problem might be that if you mention "glass door tracks", then they immediately assume you are talking about external house doors or perhaps even mirror type sliding doors for wardrobes. It might help to specify that you are looking for cabinet or small cupboard sliding glass doors.
If that does not help, here is some info direct from the Bunning on-line catalogue for the product that Adam mentioned:
Model Name: Glass Track Plastic Cowdroy 1800mm White.
Model Number: P02626