Parts for Mastic windows

We have a problem with the vinyl windows in our condo. On the upper part of the window there are two spring loaded plastic "prongs" (one on either side) that guide the window as it is slid up/down. pulling back on the spring loaded piece releases the upper part of window for removal. One of the "prongs" is broken and i need to replace it. Not being the orig, owner I looked up the brand. All I have found is the name MASTIC. You answered one inquiry about Mastic windows by giving the PVC industries web site, which lists local distributors for homowners; I called the nearest one and was told that the distributor was incorrectly listed and was only a wholesale dealer. Also, there are 5 or so mastic window mfgs in the area and they could not help me any further. Is mastic a brand name or a generic vinyl cladding technique? PVC has no address on the web site.

This is just a small part but tracing its availability is becoming a major operation. would be grateful for anyhelp.