Since that was, I believe, the first year, odds are that any part is going to be pricey unless the person is unaware of what it is. Since they were basically a fastback Coronet, you might see if you can adapt one of those. The best likelihood is to find a project/parts car on e-bay with a good taillight and buy it, take the parts you need and sell the rest. On a situation like that you would probably break even in the worst case.
That's a tough one, if you do fine one that hasn't been hit you can usually bet the plastic has started to degrade and crack. I had a 67 back in the 80's that had a perfect lens and people offered me good money back then to part with it. $400 is a lot of money but probably fair if it's a perfect lens. There is a way to mold your own replacement lenses and I think I seen an article posted here about it awhile back, a lot of work I'm sure but maybe an option? I wish I would have kept the 67, 383/4speed/power windows