We ran across a great resource just today and have added a link in our Primary Sources Guide. The site we discovered is a library guide done by a librarian at Marist College. She has done an outstanding job of providing links to a wide range of 19th, and even some 18th century, periodicals that have been digitized and are freely available online.
(These titles are all long out of copyright and thus in the "public domain," meaning they can be freely scanned, posted online etc. Copyright does protect many periodcals and books, going back to titles published in the 1920s, which is why many print materials may still be of value, as they are were not created in digital form, and are not freely available to be made so.)