The biggest problem is the plastic degrades from heat/UV. It is the only exterior plastic part that takes that sort of a beating, so about the only source is an older Tbird which has been covered.
It is possible to repair what you have with plastic filler (e.g. bondo or JB weld, epoxy, etc.), but its never going to be as good as new.
Beginning to look like we, the sccoa crowd, are going to need to make our own production run of these parts. Any takers out there?