"Back in the day" as they say, rather than saving copies of TV appearances, companies would erase the tape to re-use.  Obviously a money-saving decision that never took into consideration historical significance of some performances.  That was the case that involved a David Bowie appearance on the BBC show "Tops Of Pop." 

One of the men in charge of taping the show kept his own copy to use as material for his resume as he was the first to use a new lens during the taping.  Check out the complete story and some clips form Bowie's appearance here. Even better news, the same cameraman says he has hundreds of other copies from other appearances on the show.   Sounds like someone is about to come into some money.