Crunchyroll, 3 weeks ago, released a list of titles affected by the ending of their partnership with Funimation.
The story was originally released in two parts in two issues of Kadokawa's literary magazine, Yasei Jidai. Finding these is likely to be a pain in the ass and will cost a pretty penny, as I imagine most copies have already been disposed of and recycled.
Japan Amazon definitely has volume 6, as you can see here, but the only available format is hardcover. I imagine the paperback version will release sometime this or next year.
They're included in the set
I bought it on amazon
You can save some money by just buying it directly from amazon japan.
夏の災厄 by 篠田節子, Natsu no Saiyaku (Summer Calamity) by Setsuko Shinoda. Translated summary from Amazon Japan: "A strange illness similar to the eradicated Japanese encephalitis strikes a suburban town. The residents panic from the government's lack of action, beginning the season of evil." About the book: "A horror mystery taking place in modern times with the early warnings of a ravaging pandemic."
Rather fitting for our current situation.
Edit: Link to book/ebook on Amazon Japan: Book, Ebook (Kindle)
The actual track is not on any of the CDs in /u/talonofdrangor's link. It is on this CD, included with the volume 3 BD. You can purchase the BD here, but I doubt any of us could afford it, lol. Alternatively, you can find the CD itself on Hikari no Akari. Some quick googles will bring you to the right place.
It is track #3.