The following scale that finds on the game pages represent an attempt to give a rough impression of the current availability of the games. There is no fancy calculation scheme, but just a reflection of my experiences when looking through sites like Ebay, GameTZ and few others. So it might happen that a game is classified as rare, because I missed occurences or I just looked in the wrong places...
All the games are out of print since more than 15 years, so they can not be bought in regular stores anymore. The regional availability of a specific item may vary a lot. For example the Apple and Mac versions were only released in North America, so they are much harder to come by in Europe.
Update 2009: The latest changes in Ebay's fee structure indicate that they change their business model towards a commercial selling point and away from being a platform for private trading. This has been a ongoing change during the last years, although Ebay had often denied it. However the latest changes make quite clear where the road is heading. For people looking for old games this has two negative effects: Since there are mainly commercial traders left, the prices for games have increased notably during the last months. And because private sellers seems to withdraw from Ebay the number of available items has decreased. So it might be a lot harder and a more expansive to get the games now in comparison to the time when I created the scale show below.
Here is the meaning of the scale. Sometimes I use half shiny icons to give a tendency ().
The item comes up quite regularly on auction sites, sometimes shrinkwrapped. Most times there is a copy available somewhere on one of the popular sites.
The item comes up occasionally, but you may have to wait some weeks or few months. Considerably fewer appearances than with the first category.
The item is quite rare and it might take you a considerable time - perhaps several months - to find one.
Very rare item. If one comes up on auctions, you will probably need to be lucky or willing to afford a notable amount of money (or both).
An item whose existence is confirmed, but practically almost never shows up anywhere.