Camilo

I also recommend the C&V forum, they know everything about this sort of stuff. Great group. You don't have to buy a subscription to look at the for sale subforum there, but if you want to post a "Want to Buy" (WTB), you'd have to. I wouldn't be at all surprised if you find what you're looking for there. I've bought several things from people on the for sale subforum, the biggest being a high end frame and fork, but also various vintage and vintage-ish parts.... , and sold a couple of items.

I used to have a saved EBay search with emailed notifications using keywords of the types of Japanese steels mentioned above (as well as Columbus and some others...) I ended up with a little-known, but highly regarded frame in Columbus SLX from the 80s for a very reasonable price, but would have been very happy to find a frame in one of the better Japanese tubesets as well. That's why i was searching for tubing names rather than bike names. You might try that too. I've bought and sold dozens and dozens of bicycle items on EBay again large (frames/forks, complete bikes) and small.

For EBay and the C&V forum, you have to find sellers who have a reasonable shipping cost to your location. It can really vary, and sometimes sellers are willing to work with you if you happen to be more familiar with reasonable ways to ship frames to (in your case, Japan) than they are. It might be a good idea to do a little research on how to best get a large box to you so you can evaluate shipping costs and/or advise the seller.