The nylon insert is seldom ever shown or listed. It's called the Latch Insert.
In the past people have gotten them by contacting Colt direct.
Another possibility is Jack First Gun Parts:
Jack First Gunshop - First in Gun Parts - Rapid City, South Dakota
Others simply make their own using a piece of the proper sized nylon weed eater cord.
Just cut off a short piece that's the right size and sand or file a >VERY< short taper on one end. You want only enough taper to JUST get the cord started into the hole.
Push the piece into the hole then cut it off about 1/8" long with a razor blade. Use a small hammer and large punch to tape it in as far as it'll go, then use the razor to cut it off to the proper length.