I would like to make a mobile phone app that contains an arrangement of chess exercises and puzzles. I have found some public domain puzzles but they are mostly fairly difficult Mate-in-2s from the 1800s and early 1900s and I would like some other content, e.g., tactics exercises, exercises for beginners, etc.
Does anyone know where I can get this sort of content in the public domain?
Otherwise I have tried to find contact information for the (usually titled) players who have written popular chess puzzle books on Amazon but it's very difficult to contact these people. Is there a forum where they all hang out?