As I feared, no such source commonly exists.
The I purpose we attempt to make our own source. Maybe we photograph our coins with counter marks and money changer stamps as best we can. Then attempt single out the stamp and attempt to make a corpus, unidentified or not. Almost like a missing kids poster (Stevex6 selling kids for coins). There for the people who contribute might find a similar mark on another coin, or a specific style of mark, on a series of coins. It is conceivable that a least a geographic area could be determined. We have a relative start date from which the coin its on was made. I know the marks be from long after the coin was made. How cool would it be to work on such a project. I know it would take years but I think we could see many differnt pictures of just countermarks in a few minutes. :-) We live for this stuff right guys whos with me? Ok Ok its just an idea worthy of mention.