The hollander number for that '66 Impala deck lid is #818. 2 dr hardtop only. From a 74 paper catalog. The earlier posters are correct, every year pretty much had a different lid, and fenders, from '55 thru '77 in many instances. The '77 to '79 began to carryover sheetmetal. I have a #743 deck lid, which is '65 Bel Air and Biscayne, only, due to the presence of only one tail light in the lid, as opposed to the Impala, which had two on each side of the deck lid.
Probably could find that lid at AZ's DVAP, Hidden Valley, Impala Bob's or perhaps CA's Memory Lane, if they are still open. That is I don't know if Memory Lane or Impala Bob's are still in business. DVAP and Hidden Valley are. The one year nature of these parts makes it certainly harder to find the correct part, more expensive, as well as finding a buyer for whatever one might have.