Lost media is any form of media (art,books, music, movies, television, etc.) that is remembered or confirmed to exist that can't simply be found on the Internet. When I say found I don't mean freely available or available to rent, for instance if a DVD copy of something exists then that doesn't really count as lost. Lost means almost no evidence even of its existence, it means you're likely not to find it unless you get other people involved in the search.
Lost media is found by a dedicated group of internet archivists committed to bringing things to light.
Some people get involved so they can find stuff that they knew of as a kid, others are involved for the purpose of preserving people's art and expression. Still others just enjoy the chase.
The purpose of this page is mostly to catalogue interesting examples I find or show searches that I'm actively interested in.
The Lost Media Wiki. This is an important resource because it catalogues a bunch of existing media searches.
The Lost Media Wiki Forums. This is where most searches really take off and there's a lot of attention on them.
The Lost Media subreddit. This isn't as active as the forum but it's a good place to look for new searches, specifically stuff gets posted here by people that either aren't on or don't know about the forum so there's occasionally more unique stuff here.
blameitonjorge. This is a YouTuber that makes a lot of lost media content although it's not all that.
LSuperSonicQ. Another lost media YouTuber.
Bobdunga. More lost media YouTube content, she does some pretty in depth documentaries.