Plastic slider for secondary double glazing

My secondary double glazing sliding windows run in aluminium track. In the base of each sliding window there is a groove into which are fitted two plastic slider pads, which sit on top of the track when the window is in place.
See here: (Scroll down for a picture of how the slider fits into the window.)
The pad has a little spigot which sits in the groove in the base of the sliding window. I have lost one of these pads and ask where could I get a replacement?
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