Plastic clips broken on many of our windows

We have windows from when we bough the house in 1998 and now many of them have broken clips on each side of the window. Seems there are 2 clips or plastic pieces per side. Attached pictures show one still in place and one missing. Any info would be appreciated as I am not even sure what to look for. I have repaired Sill guides before on other windows and would like to fix these as now the windows get lopsided and do not slide easily. On a few they shifted enough that the balance is no longer connected.