The problem being that availability is a function of popularity and age. If you want something older that isn't a celebrated classic, it can be a lot more difficult to access...
But that's rarely a case. There are people maintain huge packages with thousands of old games. I can only think about maybe one or two titles that I couldn't find on the internet after some search. Besides, if a game is really obscure, chances for a legal release are close to zero.
In some piracy circles and torrent trackers, sharing rare/obscure titles is a way of gaining rep, so at least there, there is incentive in finding and distributing the un-celebrated classics.
