Erm, the release date is on AliceBooks store page itself (2016-03-19) and it was on sale just last week. In fact, this is the second time it's been sold out. I was considering getting it along with some other books, but I was already spending a lot and decided against it. Kinda regret not doing it, especially since I just got a notification that shipping charges have been increased by 500 yen.
If I'm not mistaken, I think Tojyo stated on their Pixiv page somewhere (this is the Pixiv page for the book, by the way: http://www.pixiv.net/member_illust.php?mode=medium&illust_id=55802891) that this can also be found on their Patreon page, which makes sense considering the huge Patreon logo on the cover. I wouldn't be surprised if they eventually started distributing it via their booth at Booth.pm.