It's in the behind-the-scenes guide to TAMB. You can get it on Amazon:
The Ancient Magus' Bride Official Guide Book Merkmal
(or search for "The Ancient Magus' Bride Official Guide Book Merkmal"; it's the one that came out this past May. There's another one due out at the end of this year). :)
Hm, that goes to the thunderstorm side chapter. I've read it in English on mangadex (https://mangadex.org/chapter/38684/1) but for some weird reason that chapter only has the first 2 pages up right now. I assume a site error.
It's a fun little story; I'm sad they didn't use it in the anime.
Link to Amazon pre-order or support your local book store.
Remember pre-ordering is a great way to show demand for this series and supports the creators.
Here is the physical one. It isn't out until Feb 23rd. It's still in pre order mode.
A huge thank you to my Mahoyome friends who provided the link and taught me how to order this. Love you! <3
Direct link to Amazon.co.jp listing
It ships internationally so you can use your address and credit card just like on the English Amazon website. Total with shipping to the US is about $35.
This volume is in Japanese only and comes with a mini booklet with a side story and a bag with original artwork.
Source from Twitter ...don't believe that image in the Tweet is the one that will be on the bag.
It comes with the Mag Garden Japanese magazine available on Amazon.jp.
About $7-ish USD for the magazine plus shipping. It will ship outside of Japan as long as the item is fulfilled by Amazon (which this listing is). Just need an Amazon.jp account which is just like setting up a western one.