Help - Bifold Door Hardware - Side hung

Hi,
I currently have bifold cupboard doors that are overlay doors. I need to replace one of the rollers, but am having problems finding a replacement part. The bifold doors are not top-hung, but rather they hang from the internal face of the door, is this called side-hung? Their tracking system attachs to the top side of the door, not the traditional over hung location. Sorry ... I'm not certain what this is called. Basically I'm trying to find out what this type of tracking system is called so that I can locate the replacement roller part. Most suppliers I have tried only sell the top hung tracking system.
Appreciate any advice. Have tried Cowdroy, they have something similair but not the tracking system I'm after, which means I might have to change the whole tracking system over for this one part.