Animeigo purchased the rights to the Shinobi no Mono films several years ago and they currently offer the first four films on DVD with English subtitles. They can be purchased individually or in a set. Animeigo has always done a great job with their translations and releases. Check out the extensive liner notes for the available films at their website.
The remainder of the films are not, to my knowledge, commercially available with English subtitles. If you know of a commercial source for these, please contact us.