Silverline Swing out Kitchen Window

Hope you can help me. Thanks in advance.

There is a black plastic piece attached to the window that allows the window to slide along a track on the window frame. If that is broken, then the window does not slide along the track and can come out at that end.

This black plastic is broken. Can you identify it? and do you have it for purchase? If yes I would prefer to get both a left and a right.

Thanks for your help. If you need better photos please let me know.