Good luck.... Last fall I made a window and French door set for my nieces
home and had no luck in finding anything that was readily available. I did
find a window at the borgs that uses them on their grids for their octagon
shaped windows. The plastic slides in a 1/8" slot cut in the grids and has
two small needle points that you push into the frame to hold the grids in.
You may be able to order them directly from the manf but HD or Loews didn't
carry replacement parts.
I ended up making my grids slightly longer than needed than beveling the top
and bottom sections about 2 deg so they would snap-in to a 2 deg cut on the
frame. Small chisel does this nicely. The grids were made from ash so they
have enough flexability to be bent slightly and inserted. To remove, simply
pull on the center section of the grid. Now if I've guessed right, the
grids should not shrink enough to ever fall out.......