Plastic corner keys??? for Sugarcreek Deluxe Model 399 1 1/4" Self Storing Doors

Hello,

I have broken some of the white plastic guides that are at the top of the storm windows in this door. I have two doors, each with two windows and need some replacement guides...a total of 8. Do they become brittle with age? If so I am planning to replace all of the guides.

White plastic piece is 1 inch long and 1/4 inch wide from side to side in channel.
The protruding piece breaks easily if the window is twisted even very slightly while removing. The piece has a screw hole in the center to attach to the window channel.

Thank you! I will order.