These are sites that I have found to have helpful research info.  In some cases, they also are stores or sellers, so I guess I should note that I don't have any relationship with any of the vendors, nor should the fact that they are listed here, be considered an endorsement of them.    

On the other hand, it's really nice and generous that so many collectors and sellers are willing to share their information!  And it's great to be able to support them with purchases, if you find something on their sites that you just can't resist    I would just recommend that as always, you do some good consumer research before purchasing.