Arrangement of chess puzzles?

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?

cant help you, but love to see such an app on my phone.

...even more i'd love to see such a phone in my custody...but with such an app, it would be even more reason to get it (finally something useful...)