"Hard to find" is a common, but fairly generic and non-specific characterization of games that are difficult to obtain. Many are out of print or only available from small publishers who may not have distribution agreements or sufficient quantities to allow their game to be widely available.
Good resources for finding hard to find games
Naturally, BGG is one good place, and you should read the Guide to Buying Games on BGG. Make particular note that you can subscribe to receive notifications when a game goes on sale or is listed for auction.
Additionally, it is often worthwhile to directly contact the publisher, especially smaller ones, as they will occasionally have a stock of games they are willing to sell directly.
Naturally "Hard to find" games turn up on ordinary auction sites (such as eBay).
Mother Goose's Attic is a source of information about "vintage" card games. It includes descriptions of games, pictures, "canned" eBay searches and sometimes the rules.
