I have found a lot of useful information on the US Secret Service E-Library Website. You have to register for it, but it has a huge database of counterfeit checks that you can search by just about any field you can imagine - remitter, bank, payee, etc.
Here is a link to their website It has been a long time since I signed up, so I don't remember how difficult it was, but it has proven quite useful. Several times I have been able to confirm that a cashier's check was counterfeit by finding an almost identical one in the database.