Certainly # made is a factor, but 'desirable to restore', and even more so is 'value when done'. If not a lot are being restored, there is just no money in it and would be a bad business decision to carry it. A lot of parts were used on different models, so the less desirables have parts available by association. If its a functional part that millions of cars used, the auto part grocery stores will have it. Then the next level gets picked up by the restoration suppliers. On the other extreme are the cars that very few are out there and their rarity makes it possible for someone to make small runs and recover there costs through what would seem like ridiculous prices to us A-body guys.