The Examkrackers 1001 books in physics, chemistry, and ochem are basically all discrete-type questions. Biology has both passage and discretes. I found the books very helpful.
there's also the site mcatquestionaday.com. It has tons of random questions on all topics covered by the MCAT. Be careful about relying on it too much though, the questions on it are easier and not asked in MCAT-style format. It's good to review topics though