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. You can also search Craigslist, eBay, Etsy, and Buyee. Digital rarities can be found on archive sites and peer-to-peer networks.
Bandcamp
Kirby Manga Mania's localization because I haven't seen it on Bookwalker
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.
osu! is being overhauled. Several non-lazer charts are cloned in osu!octave's data.
plug-and-play consoles not in MAME's database
pubnix/tilde servers
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 the pedophile Bob Magrann.
Archie's Sonic Universe compilations
dedicated/LCD handhelds
Discord logs with DiscordChatExporter
DLsite was blacklisted by several payment processors and some of its games have been deleted. Justified for material considered illegal overseas, though the SFW sections (home, soft, comic, app) probably should be mirrored.
Internet Archive because INTERNETARCHIVE.BAK is paused at the moment
Neocities is fine at the moment, though it could become a good source of modern history.
Paramount+ delisted The Fairly OddParents: Fairly Odder. It's probably not a big loss, though should we download other streaming exclusives just in case?
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 disappeared.
Valve seems to want you to think Steam and SteamVR will exist forever.
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.
DVDs aren't as sturdy as Blu-Ray Discs, though MakeMKV extracts most of their contents. Menus and galleries can be copied with a capture device such as the ClonerAlliance Box Pro.
itch.io is a mixed bag, though some of its better games aren't mirrored elsewhere in case it goes down again.
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, but I just want to be sure.
Laserdisc/LaserActive games with Domesday Duplicator
New arcade games aren't as easy to find online.
OpenLeaks and WikiLeaks
The USB-GDROM's translated guide is down, but the original one is still up.
The first Xbox's DLC has become scarce after the closure of the original Xbox Live.
Corruption can be easily mitigated if a partition is formatted with ZFS, btrfs, or HAMMER2.
Peer-to-peer networks such as DC++, BitTorrent, and eDonkey can keep files in circulation without depending on a particular server.
rsync clones files as closely as possible, while RAID1 copies an entire device.
A storage unit can guarantee physical media is kept dry and cool.
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, though I don't know of any emulator that can run them.
ld-decode to prepare Laserdisc rips for compression
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.
Some plug-and-play consoles still have preliminary status in MAME.
I downloaded The SCP Foundation's collection, but I am still researching ways to share all 6.5 (!) gigabytes.
Tamagotchi Pix's firmware has been dumped, yet MAME cannot boot it yet.
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.