Plastic slider

That looks like the shoe found on the ends of the arms for our 39-312 operator. This shoe is unfortunately no longer sold separately, though. We can only sell you the complete operator. I am sorry for the inconvenience. 

There's also no guarantee that the 39-312 is what you want. This particular design was used on a few different parts, if memory serves. But the 39-312 is still a good place to start.