I had the same problem and never did find a suitable answer. Fortunately I was just trying to save money and eventually just bought a new lens from Ferrari UK, who had them at the time. I believe you are correct in saying that they no longer have any but have you checked? Ferrari UK often suddenly find new old stock in their warehouse. I have been tehre and it is a huge aladins cave.
I had bought a white/white lens off of ebay. I then tried to tint it but could not find a way to do it. I tried using some 'yellow' headlight paint - they used to have to use yellow headlights in France and drivers and tourists used to paint their headlight bulbs yellow to get the effect. After France stopped doing this I kept some old paint I had - but when I tried it the effect was too yellow. I then bought some orange see-through paint that is used in decorating ceramics, you can buy it in arts and crafts shops. There are a range of colours and I tried mixing up a colour to match the lens on the opposite side. I was not very successful andd it is really hard to paint on because you can only get to the inside of the lens through the side light bulb apperture using a long paintbrush. I ended up with a very streaky orangish lens - no use at all
I gave up and bought a new one.
Actually the white/white lenses can look good depending on the colour of the car. I would suggest you buy a pair od these and then monitor eBay until the right ones turn up. You can always sell the white/white lenses again.
1977 400 Auto
1976 365 Boxer