From the window that the balance supports
A home owner from Coral SpringsFlorida says:
April 6, 2016
One of the pieces broke and the balance just slides by the window. I cannot find a place that sells replacements. The one in the picture is pretty beat up too. There is one on each side of the window.
Paul from SWISCO responded:
April 8, 2016
Please see our 18-015 top sash guide for PGT windows. Let me know if this has the same measurements as yours. I think this could be the part you want.