Where can I find EXTREME Chess tactic puzzles

Very hard Puzzles that require me to calculate 6-15 moves

Any websites?

lichess.org and put setting to hard?

I believe if you have an actual computer, Informant offers free downloads of their tactics and endgame sections of their Informant game collection books. Some of those positions are very deep with multiple variations. Also, on Android, there's an app called Perfect Chess Trainer, PCT, it's only a few bucks, and there's a huge tactics trainer section. Many of those are very complex with playable variations.

            You mean, like THIS for chess.?                                                                                                                                                   http://www.wtharvey.com/

Buy Chess Informant's Encyclopedia of Chess Combinations in book or CD format. If that is even too easy for you, search for puzzles used for testing chess engines.

GM Alejandro Ramirez recommends composition studies after you've mastered tactics puzzles. They help build creativity.