Where can I find this?

It's available from Sinister Cinema (sinistercinema.com). This is the Americanized version, Half Human, with inserts starring John Carradine and Morris Ankrum, and Carradine narrating the Japanese sequences.

Unfortunately the original Japanese version (Ju jin yuki otoko, 1955) has been locked away in Toho's vaults since the 1970s owing to protests from the Ainu of Hokkaido, who claim the film maligns them. Having never seen the Japanese film, and since the US version has no visible anti-Ainu sentiments, I can't say what it is that the Ainu object to.

It appears highly unlikely the original will ever surface again (it's apparently unavailable anywhere), so the US version is the closest we're ever likely to get to it. But even this edited version is pretty good, making the unavailability of the original all the more frustrating. I first saw this movie as a kid and have always liked it, so I recommend it even though it's not the original, unadulterated, complete Japanese film.