A lot more 40 year olds have come out in the year since this original post. But they are all amazingly expensive. You should be able to find these online:

Highland Park 40 (very highly rated)
Bruichladdich 40
Auchentoshan 1965, 40 Year old
Glenfarclas 40
Bowmore 1955, 40 Year old

There are also a slew of Macallans over 40 years and too many independant bottlings to even start to list.

But generally speaking, 40 year old whisky is past it's prime. They just get too woody after that many years in the barrel. There are exceptions, but you stand a good chance of paying $2000 for a OK whisky. So be careful and read lots of reviews before you purchase somthing this old.