This is probably a dumb question, but I just want a sanity check…
In theory, the painting “The Great Wave Off Kanagawa” is at the Met in NYC:
http://www.ibiblio.org/wm/paint/auth/hokusai/
I’ve gone to the Met a couple dozen times and have not managed to find it. Does anyone know where it is?
I’m pretty sure I saw iton a greeting card in the gift shop. Or do you mean the original? 
It’s listed as being in the permanent collection, so it should be there somewhere. It’s small, only 10 inches by 15 inches, so it’s probably easy to overlook; perhaps you missed it somewhere? I nearly strolled right by American Gothic in Chicago, because it was so small and unassuming, and the Great Wave is quite a bit smaller than that, maybe a quarter of the size.
And of course there’s always the possibility that, even though it’s at the Met, it could be “off display” for one reason or another.
Ha. They actually have a big poster of it in the gift shop alcove right near the Asian area. And no, they don’t know where it is either.
And further research reveals it’s actually a print that the Meta has, not a painting. But still…