I believe @genchiro found some on their Japan journey for about ¥40,000 or about $290 (a pretty good deal IMO). The whole journey was documented here:
If you’re in Japan, I believe they should have stamps – the main reason stamps were so hard to find outside of Japan is that shipping Japan post stamps internationally is a legal/customs gray area (luckily I did get stamps in my box even though I ordered to the states through eBay earlier last year).