Which window part is this

Hello,

If you're referring to the white plastic piece, that would be the top sash guide. We may be able to supply replacements but we would need to see photos of an intact guide removed from the window to compare to our inventory. 

It sounds like you may also be looking for the channel balances used to support the weight of the window, which are held in place with the top sash guides. We would need to see an original balance uninstalled as well to narrow down your options. See the videos related on this page to help you through the process. Thank you!