Classic Industries sells all the lenses.
1966 All Makes All Models Parts | MA1836 | 1966 Barracuda - Bottom Tail Lamp Lens - LH (Driver's Side) | Classic Industries
So does Laysons
http://www.laysons.com/Catalog.cfm?nCatID=307&nModelID=3&nModelYear=1966
Both options aren’t cheap.