I'm sure there must be some NY dealer somewhere with his prints, and if you find anything over there, I would really love a little report! Even in Canada, where he worked for most of his life, he's pretty much unknown outside of artistic/academic circles. There has been no recent publications of his work since the early 80s. Some exhibition catalogues, but no proper reprints of his books. It's a shame.
It's the Velvet Underground syndrome: only 10 people saw his work, but the 10 were all photographers, and his influence is actually bigger than his popularity. He was buddy with Robert Frank, Leonard Cohen, Frank Zappa, Andy Warhol, and plenty of other people.