Obviously the best way to buy Japanese baseball cards is to travel to Japan and visit some card shops. But since that's not necessarily easy to do (especially in the middle of a world-wide pandemic), I thought I'd list the best ways to get Japanese baseball cards from the US.
1. The absolute best way to buy Japanese baseball cards is to use a proxy service company such as ZenMarket, JAUCE or Noppin to bid on stuff on Yahoo! Japan Auctions and other auction sites as well as order items from on-line stores that do not ship to the United States. There is a wide variety of cards for sale on the auction sites, often quite inexpensively, so that even with the fees that you pay to the proxy service you're still getting a good deal. (NOTE - there are other proxy services like Buyee and Janbox but I haven't used them. The three I list here I have used and endorse.)
2. Mint is a chain of card stores in Japan. They have an on-line store called Mint-Mall which will ship to the US. They have single cards as well as unopened boxes for sale here. I have not used this service but just playing around with it a little bit it looks like you will pay at least 2500 yen for EMS shipping to the US on whatever you buy. That's prohibitively expensive for buying a single card but probably not too bad if you're buying an unopened box. The site lists items that are for sale in various Mint Stores - items added to your cart from separate stores will be treated as separate orders (and each charged a separate 2500 yen shipping fee).
3. Mandarake is a chain of antique toy stores in Japan that carries vintage baseball cards. Like Mint, they have a website through which you can order items to be shipped to the US. They do not carry new product but they're a great place to check out if you're looking for Calbee cards from the 1970's and 1980's at pretty good prices. This post describes how to order from them.
4. There are many Japanese cards available on Ebay although the selection and prices can be very inconsistent. Actually lately there's been at least one seller whose prices are somewhat delusional.
5. Similarly there are a variety of Japanese baseball cards available at both COMC and Sportlots but the selection there is also very inconsistent and some of the prices are completely out of line with reality.
6. There are two US based card dealer who buy and sell Japanese baseball cards - Rob Fitts and Prestige Collectibles. The only reason they are last on the list is that neither one lists much of what they have for sale on their websites so if there is something that you're looking for, you'll have to ask them about it.
8/10/21 NOTE - I was way overdue at updating this page. I wrote the original version of this page in 2013! Several of the websites I reference no longer exist or no longer sell cards. I'm also not sure kuboTEN is still in business - unfortunately even if they are there are much better options for proxy services available.
1. eBay - eBay is probably the best place to find Japanese baseball cards although it is very inconsistent - you never know what you're going to find. And, of course, in many cases you have to deal with the frustration of being out-bid on something. But for volume and variety, it's tough to beat what's out there on eBay.
One suggestion that I would make - if you buy something from someone who is actually located in Japan, ask them if they are interested in selling other items to you. You might be able to set yourself up with someone who is willing and able to go to a card shop in Japan and find things for you. I've had this happen twice now over the years - the first was with an American who had access to a military base so we could ship stuff to each other via US mail, the second was with the Japanese gentleman that I get my sets from now.
2. On-line Japanese card shops - there are two of these that I'm aware of that will ship to the US - AmiAmi and Rakuten Global Market. Pretty much all you can get from these are new, unopened boxes of cards. I've ordered a bunch of box sets from AmiAmi without too much trouble (although they appear to sell out of stuff quickly) and they offer both EMS and SAL shipping. I've only ordered a couple of things through Rakuten - I could be wrong but it seems like they only offer EMS for shipping which can really run you.
3. US card dealers - the big US card dealers are Rob's Japanese Cards and Prestige Collectibles. Rob's is run by Rob Fitts who is focusing more on writing great books about Japanese Baseball history so the business is focusing more on older cards than on current cards - although I think he gets in at least a couple of the BBM 1st and 2nd Version sets each year. Prestige Collectibles used to be run by Gary Engel, author of "The Japanese Baseball Card Checklist And Price Guide" but is now owned by Robert Klevens, another of the Japanese Baseball Card "pioneers". Prestige Collectibles also runs auctions every so often with a lot of very interesting vintage stuff. I wouldn't necessarily take what's on the two websites as gospel for what either of them have in stock - if you're looking for something specific, ask them.
4. Auctions Yahoo! Japan via kuboTen. kuboTen is a proxy service that will bid on auctions on the Yahoo Japan site for you. There are fees associated with this and you'll end up paying for shipping twice - once for the seller to ship the item to kuboTen's office in Fukuoka and agin for kuboTen to ship the item to you - but a lot of times you can find great stuff fairly cheap on Yahoo Japan. Be patient with kuboTen, however, as it's a one man show and Craig has been known to get overwhelmed at times.