White plastic that holds in glass pane

Thanks for posting. This trim is called glazing bead. We may be able to help, but we'll need you to remove a sample of the glazing and show us the 'end profile'. That cross-section view, along with dimensions, will help narrow down your options. In the time being view our selection of glazing bead here. 

Thanks for getting back to me. I am afraid we don't have an exact match for this glazing bead, but check out our 55-191 or 55-194 to start. Compare their 'end profile' dimensions with your old glazing and see if you think one of these might work as a substitute. See what you think.

Profile view of 55-191

Profile view of 55-194