Here is the link for RightStuf:
http://www.rightstuf.com/1-800-338-6...m/104569/4/0/0 Unfortunately, it seems like they are no longer taking preorders.
Anime Castle still has preorders open:
https://www.animecastle.com/p-152436...-story-hc.aspx I don't have any experience with Anime Castle, but it has good reviews and people seem to be satisfied with it.
Amazon in the UK also seems to be selling it and you could probably find it there with a simple search. It's not on the US site, though. Aside from that, the only other place I've found the novel on (AnimePoko) has been labeled a scam site (but that could be incorrect info, as I also have zero experience with them).