I've been doing some research on counterstamps using a couple of websites that have large databases of old newspapers. The two sites I'm using are Fultonhistory.com, which has mostly New York City and New York State papers, but is expanding, and Newspapers.com, which has papers from all over the country. The former is free (although donations are welcome); the latter is not.

I'm wondering whether there's something comparable with old magazines. Maybe some readers might know?