The Kaplan book is good, but unfortunately you will not find that many practice questions there. The MGMAT archive might serve you better.

The Challenge archive is overkill for most people - and I say that knowingly since I work for the company! At least half of the problems are what we call "900 level" problems, which means they are harder than anything you'd actually see on the test. The question banks have a mix of easier and harder problems but they are mostly within the realm of what you might see on the test (though, even there, there are a few that are a bit much).