It's just not that popular of a lens so it'll be hard to find. You just have to keep checking those sites. Why does this lens appeal to you specifically?
It is a quite old design. I found one a few years ago in Germany, tried it and didn't buy it. Almost any other more recent design is a better choice (the last two Tamron designs, a Sigma, Nikkor old or new), providing 1:1 magnification, and better IQ.