Little white plastic piece

Thanks for writing in! The white plastic piece you are searching for is the face guide. This piece isn't necessarily critical for the function of the window but you may notice that the sash is a bit loose in the frame. 

I'm sorry to say that we would not be able to supply you with a suitable replacement for your existing guides. Unfortunately, the guides we do carry would not work as a suitable replacement for the one you have.

Our sincerest apologies we could not be of more help but please don't hesitate to reach out with any other hardware repair projects you might have.