I have seen nos lenses on eBay
There were probably 100,000 67 darts made so backup lights should still be findable.
I have some that I might be able to part with but it will be a while till I know.
The car my buddy and I bought has a bad bumper I don't know how the backup light bezels are. I found a usable bumper and 1 of the bezels is ok, the other is broken but usable.
MA3676 - 1966-67 Dodge Dart; Back-up Lamp Lens; LH or RH; Each