Kaplan's Qbank is probably your best question bank. Otherwise, here are some other possible sources:
1) Board Simulator Series (Ebay)
2) Kaplans Qbook (Bookstore, online)
3) Appleton & Lange (Bookstore, online)
4) NMS (Bookstore, online)
You can get the NBME exams from the website (
www.nbme.org). They're $45 a pop and are probably the BEST predictor of your actual score on Step 1. Do NBME-1 early on in your boards prep to see where you stand and then do the second one shortly before the exam.
Whatever you do, make sure you do at least one of the NBME exams. There is no reason to blindly walk into the real exam not knowing how you'll do(unless of course you don't have $45).
Hope that helps.