The original intention, as I understand it, was that Cellar 13 would be available only at the distillery. At the last minute, they also decided to make it available at airport shops, but its cachet is that it is supposed to be hard to find.
Also when I was at a tasting of Glenmorangie products, Anthony Burnett, a company director told us that they had a lot of difficulty with the actual name and the bad luck associated (in some peoples minds) with the number 13 and that a lot of retailers were unwilling to stock it. It's a shame because I really liked my bottle and would like to replace it.
According to their website, the duty free shop on the Canadian side of the Detroit-Windsor Tunnel stocks Glenmorangie Cellar 13. If this is correct, it would a bit more widely available than just airport shops, though it doesn't help North Americans far from the border.