Sometimes I'd like to purchase computer games which are relatively old, such as Maxis' SimCity 3000 or the original make of Medal of Honor, but the games themselves aren't available through typical sources. Sometimes you get lucky and the game's available on e.g. Steam, but sometimes things appear simply unavailable. Sometimes, it's a game I once had a CD for when I was little, but at present time I'm unable to find the installation media.
Oddly enough, Googling for the game gives several illicit sources for various games, but I'm not interested in doing things illegally.
What kinds of ways might I use to increase my success rate when looking for such "abandonware"?