You can purchase this in Norg - the price depends on your Tenshodo fame I think.
I got a linkshell member with high Tenshodo fame to go and get it for me, just Mog-fedex'd it back to me in Kazham, so I didn't have to try going through Sea Serpent Grotto.
I think a couple of the other Great Katana are bought from the same guy. She said there were several on sale there.