sears window pivot shoe replacement part needed

We have Sears windows and a plastic part is broken. It's the part that slides along the side of the window and keeps it in place when sliding the window up and down. It broke in half, so my picture only shows half; the other half is still in the window. I'm not sure what it is called. Thanks for your help. I need at least 2 but may buy more for future problems. Sorry the pics are blurry. Thanks.