Reddit mentions of The Art of the Japanese Sword: The Craft of Swordmaking and its Appreciation

Check out the wiki for many links and resources on the Japanese sword. I recommend you learn as much as you possibly can before attempting to purchase genuine nihontō. Read at a bare minimum The Art of the Japanese Sword. Many retail dealers in Japan will not sell to just anyone; a little bit of knowledge and respect will help show that you are serious about owning a genuine sword and are aware of the proper etiquette and maintenance required. Speaking of which, watch these two videos by nihontō book dealer Grey Doffin on sword etiquette. Committing a faux pas in the presence of a dealer is a very fast way to ensure they will not sell you anything. EDIT: oh, and dress neatly. That kind of thing matters.

Be aware that de-registration and export paperwork for nihontō takes several weeks and fluent Japanese language abilities. Therefore you will likely not be able to simply pick out a blade and fly home with it. Instead, the dealer will take care of paperwork and ship it to your home address, usually via EMS.

What is your budget? You should understand that an antique katana, polished, in shirasaya (no full mounts), papered by the NBTHK to Hozon level (the baseline), will usually cost several thousand dollars minimum. It goes up (way, way up) from there. I strongly discourage you from buying any antiques that do not come with NBTHK or NTHK papers, because unless you are intimately familiar with collecting and appraising nihontō (as it is evident you are not) then the onus is completely on you to know the quality and value of a given piece. There are plenty of overpriced, flawed, gimei (false signature), or other pieces that a dealer will happily unload on an unschooled gaijin. Papers will let you know that it is at least authenticated to a certain base quality level and specific school or smith.

An exception would be shinsakutō (newly-made blades, i.e. those from the 50s and later). These will as a general rule not be papered. However they will also have a higher baseline quality level than the worst antiques because of Japanese laws regarding licensing smiths. Unfortunately a newly-made blade (especially from the past 30 years) tends to be particularly expensive, with a polished katana in shirasaya tending to be $10,000 minimum.

Of course the sky is the limit. There are Tokubetsu Hozon papered antiques for $10,000–30,000, and Jūyō / Tokujū antique blades from $30k to hundreds of thousands of dollars. There are also shinsakutō by mukansa (above contest) smiths today that will run you $40,000+. But I am assuming that wasn't your target budget.

Where in Japan are you going? You may want to check out the websites of some of the dealers beforehand to get a sense of what they keep on hand, especially the ones that have some sort of English capability. There's a list of commercial dealers on this page though of course that's just an international sampling.

I don't know, what else can I tell you… the Dai Tōken Ichi (biggest Japanese sword dealer show in the world) is November 1–3 this year… also, visit the NBTHK museum if you'll be in Tokyo. Oh! And understand that you can find mogitō (decorative swords) and iaitō (training swords for iaidō) in Japan, some of them very expensive (e.g. $1,000), but these are not real swords and they have aluminum-zinc alloy blades. I would not spend a lot of money on such an item. Maybe $350 or so on an iaitō as it will look exactly like a real sword to anyone who isn't a collector, won't require maintenance, you can fly home with it, train forms with it, etc.; much better than a "wallhanger."

Regards,

Gabriel