Well, of course I can suggest Grockit, but I'd also use the vast BTG library of blogs for questions, as well as the SC forum. You can do a search for a specific skill tag, like "Probability" and then copy/paste 10-15 Prob questions into a Word doc. Then go over them, and review the answers when you're done. That's one way to take advantage of all the free questions out there!
If you've already done the OG and MGMAT SC, you should be doing okay.
