I don't think you can buy Tamahagane from Hitachi, unless you are a certified Japanese Sword maker. I also don't believe that Dictum gets it from Hitachi. The pictures they provide show a very small scale operation of Tatara-buki, not the large-scale preparation by Hitachi.
Anyway, there are small shops where they also run Tatara-buki at a small scale, and you can buy theirs. It would be around $100 per Kilo. Alternatively, you may want to participate in public events often taken place at various Japanese swordsmith to make a knife or something with leftover Tamahagane.
Hope this helps.