What books have you completed? The OG? Powerpreps? Kaplan 800? Powerscore CR? MGMAT Number Properties? Difficult material is out there, but you shouldn't skip the medium-level questions, either.

Have you made an Error Log for incorrect questions? Even if your skill level is very, very high, remember that reviewing incorrect answers and mastering concepts can actually be MORE valuable than simply answering the most difficult questions you can find.

If you're on Grockit, you can also create a custom-game with only 16-32XP level questions from our quiz bank (our highest difficulty level).

There are also a lot of unofficial "difficult" materials posted here, GMAT Club, URCH, etc. but be cautious with some of that as it's usually student-created and not always reliable.

Successful students stress quality over quantity. It isn't just how much you study, but HOW you study that counts.