Take a peak on Reddit's /r/datahoarders or /r/datacurator. There's also https://wiki.archiveteam.org/ who may point you in a direction.
Sometimes stations do keep a record of their old broadcasts. But that depends on their size, organizational structure, funding, etc. some stuff is just lost to time simply because someone felt it was cheaper to just reuse tapes.
As for value, that's hard to determine. Depends on what was taped, when, by whom and in what kind of context. That's where curation comes in to play.
Sometimes stations do keep a record of their old broadcasts. But that depends on their size, organizational structure, funding, etc. some stuff is just lost to time simply because someone felt it was cheaper to just reuse tapes.
As for value, that's hard to determine. Depends on what was taped, when, by whom and in what kind of context. That's where curation comes in to play.