small square plastic piece broke that slid in upper track

Hello,

We offer two rollers that may work for you as a replacement for yours, either the 23-196 or 23-027. The main difference between them is the design of the housing, which I can't make out from your photo since it's turned still installed.

Look over both of these parts to get started to see if you can confirm the right match. Please note that you may need to remove your original roller to compare the specifications. Thanks for posting!