Dude, you said 3 days ago that there was one in your LFS.
I don't think there are any reputable suppliers that sell nautilus to the general public, but they do seem to show up in LFSes occasionally. Even if there were, TONMO members, including professional researchers and public aquarists who have worked with them, pretty much are unanimous in thinking this is not an appropriate animal for private keeping except possibly with the exception of the most advanced private aquarists there are. It's also not clear if nautilus is at high risk for overcollection in the wild, although if it's survived the high demand for its shells, I doubt the private aquarium demand would make it significantly worse.
Anyway, if there is no longer a nautilus in your local fish store, there is really no standard place to seek one out, and I don't think anyone would encourage you to do so anyway. From your report in another thread, you say you are willing to keep an octopus in an uncycled tank despite assurances from a number of people here that this is likely to kill it. Robyn, who is a professional marine biologist studying nautilus as a career, has said:
Please please please do not purchase that animal.
which is, well, really not very vague at all.
Although I assume marinebio_guy and gjbarord are not completely opposed to private keeping of this animal, because they wrote an article in TFH magazine about it, if you didn't refuse to buy the article, you'd find that they recommend a huge tank (3' x 1.5' x 2' for a single