This is a supplement to the Archive Team's Deathwatch and Fire Drills. Most of these are just ideas I had jumping around in my head, and they might be subject to changes.
Physically, there might be retro-themed stores with vintage media and hardware in your county of residence. Remember to check Craigslist, eBay, and even Buyee. Digital rarities can be found on archive sites and peer-to-peer networks.
I usually discourage mirroring the works of outlaws, though I will make exceptions for evidence that has been covered up, such as Justin Silverman's continuous support for pedophile Bob Magrann.
Archie's Sonic Universe compilations might be on MyAnonamouse.
dedicated handhelds
Discord logs with DiscordChatExporter
DLsite was blacklisted by several payment processors and some of its games have been deleted. Somewhat justified for material considered illegal overseas, though the SFW sections (home, soft, comic, app) probably should be mirrored.
DVDs aren't as sturdy as Blu-Ray Discs, so extract their contents with MakeMKV.
Internet Archive because INTERNETARCHIVE.BAK is paused at the moment
itch.io is a mixed bag, though some of its better games aren't mirrored elsewhere in case it goes down again.
Kirby Manga Mania's localization because I haven't seen it on Bookwalker
Kiwi Farms documents many online criminals, yet the owner is prone to deleting posts and threads it dislikes. Some of the pages were mirrored on archive.today, though I just want to be sure.
Laserdisc/LaserActive contents with Domesday Duplicator
Neocities is fine at the moment, though it could become a good source of modern history.
new arcade games
obscure Fandom/Miraheze wikis
Onion Farms is the primary competitor of Kiwi Farms. Some of the moderators are goofballs and the whistleblowers aren't as active as the ones on other forums, though it occasionally comes in handy when Joshua Moon poops the bed again.
OpenLeaks and WikiLeaks
osu! is being overhauled. Several original charts are cloned in osu!octave's data
Paramount+ delisted The Fairly OddParents: Fairly Odder. That show is probably not a big loss, though should we download other streaming exclusives just in case?
pubnix/tilde servers
SocietyGlitch is a private torrent tracker dedicated to rare documents, photos, and videos. While some of its media have been mirrored on Internet Archive and reviewed on Oddity Archive, others could become even harder to find if the primary source shut down.
Valve seems to want you to think Steam and SteamVR will exist forever.
The first Xbox's DLC has become scarce after the closure of the original Xbox Live.
yt-dlp gets stuck when trying to process an age-restricted video and some music videos, though it can be supplemented with YouTube Video Finder.
Android's apps and games are common outside of the Google Play Store and Amazon App Store, though emulators are hit-or-miss.
Dreamcast and NGPC connectivity
Famicom Disk System's QD dumps are more accurate
ld-decode for Laserdisc rips
N-Gage games have been uploaded to Internet Archive, though EKA2L1's compatibility is mediocre.
Many online-only games have been adapted to other mediums.
I downloaded The SCP Foundation's collection, but I am still researching ways to share all 6.5 (!) gigabytes.
VMU-to-Dreamcast communications are not properly emulated yet, meaning that the Chao Adventure games can only send powered-up Chao to the Sonic Adventure duology on real hardware. However, DreamPotato seems to be progressing well.