Plastic block that is attached to a spring in Imperial window

Hello, thank you for posting,

Unfortunately these channel blocks are not available separately. The full channel balance will need to be replaced. To further review, we will need to see an undamaged channel balance, where we can see how it installs at the top and any attachments at the bottom or end of the pulley cord. 

Please also provide the length of the metal channel along with the number/letter stamped on the metal. From there we can determine the series needed for your window.