You need to join a Japan ordering proxy service. I use www.fromjapan.co.jp for all my hard to find parts. The site searches multiple Japanese sites at once including Yahoo auctions Japan and Amazon Japan. I have found lots of parts for my 1989 TownAce and I've seen seats on there several times. It wouldn't be cheap to ship a full seat from Japan, but it's possible. My guess is the seat itself would cost you around $100 with the proxy fees included and the shipping would be at least $200...probably more.

You pay for everything with PayPal which automatically converts USD to JPY. A little tip to help you find more parts. Go to Google Translate first and translate "TownAce" and "Seat" separately into Japanese. Then before you press enter in the search box make sure the drop down menu is set to search in Japanese (instead of English).
I saved you the trouble this time, but use the technique for future searches. タウンエース シート

Also I forgot to mention to verify the seats will fit in your van (besides visually matching them) look for the code CR10, CR20, CR30. That is the van series similar to our TownAce's and will most likely bolt in. For instance my '89 is a CR32. However I have not yet found a perfect way to make sure which code is best for my can. I believe that last things I bought (headlights and tail lights) were listed under code CR20.

Once you see a seat that looks correct, click on the listing. The description will be in Japanese. Click "translate" to see it in English. If you're already registered with the site simply click "buy now" or "bid" to buy the item. You will receive instructions from the website on how to proceed. It may seem a little overwhelming at first, but honestly after your first purchase it will all make perfect sense. This site is a life saver for all my JDM parts.