I look on a range of Russian websites including VK, eruditor, twirpx, piratebooks.ru, as well as sites like pdfdrive, oceanofpdf, ebook-hunter.org, sanet, torrent files sites, archive.org (there's a whole process of setting up calibre with de-DRM tools, editing the archive link to download the acsm file, downloading it in an older version of Adobe Digital Editions, then ripping the file that Adobe produces...), Scribd, and occasionally IRC.

There are the obvious sources like Zlib and LibGen which is where I start from, unless it's a particularly old or rare book.

Nothing I do is particularly special. It's just time consuming and it takes a bit of familiarity the quirks of each site, signing up burner accounts with some of the websites, and prioritising sites based on the likelihood of which will have the ebook in question. For example, that request for Chernikhov's Architectural Fantasies wasn't appearing in any of the ideal sites and due to its age and being a Russian book, I found a Russian-language copy of it on VK by searching for the Russian title of the work there because VK, due to their userbase, has a strong inclination to host Russian ebooks and I figured that the person requesting it would be most interested in seeing the illustrations so in this case it worked out.