The problem with transparencies, even if they were actually used in the production of the comic, is that there could have been many of them printed. And of course, as others have mentioned, you don't know if someone just created it by themselves so they can sell it on eBay.
Sometimes transparencies were used to modify the original art without actually modifying the art itself. They might do this if they wanted to emphasize some lines or add in some item when they are at print. But, these will usually come with the original art and probably still taped on.
Then, there are color separations that have three different films stacked on top of each other, which is then stacked on top of a copy of the original art. Nobody does this anymore though.