You are lucky then as you can use the more popular one. They made 2 Trunk Lids in '66. One for the Impala 2-Dr HT and all others. You can use a Lid from any 2 and 4-Dr Sedan, 2-Dr Caprice, 4-Dr HT and, of course, another Convertible. You can even use one from a Bel-Air or Biscayne. The only difference on the Biscayne is that you would need to drill holes for the Impala Molding.
And although I have a few left, unfortunately all the good ones are gone.