The only place that mfg the correct "mylar" is this place.
Precision Premium Automotive Glass Parts | Automotive Parts
Lots of retailers sell their stuff. Try Nigel at National Moparts here in Ontario.
I have some leftovers from when I swapped to stainless. The stainless corner pieces just rtv on. They don't snap or clip in. I may have some if need be. Make sure you use 67 style gasket.