You're right that a lot of test prep company materials don't have as many of the "elegant tricks" that the official question have. That's why the best source of practice questions is the official test questions themselves. (We are working on that, by the way - my company is, at least.)
I'll assume you already have OG13 and the Quant and Verbal supplements, 2nd edition. You can also get OG12 and the 1st edition supplements - most of the questions are the same but some are new.
Within GMATPrep 2.0, there are 90 free questions, 15 of each type. You can also buy Prep Pack #1 from within GMATPrep, which will give you an additional ~200 quant and ~200 verbal at varying difficulty levels (as well as 24 IR questions).
You can also use GMAT Focus (
www.gmatfocus.com), which are adaptive mini-CAT quant sections (24 questions rather than 37). These are adaptive, so you should hit whatever questions are in your scoring range. (Note: there is a limit to the number of times you can do GMAT Focus without seeing repeated questions.)
All of the above adds up to a couple of thousand questions. That should be more than enough, assuming that you aren't just blowing through all of the questions but you are actually taking the time to learn from each one. :)
Finally, one important thing. Always define "harder" as "harder for you." It's not worth it to go study the hardest questions out there UNLESS and UNTIL you really are capable of tackling all of the questions below the top level. You won't see the very hardest questions unless / until you're good enough to lift yourself to that level. So just remember: "hard" really means "hard for you," whatever your skills and scoring level. That's where you concentrate!