Quote from murphyle
I agree with Legz: get the ENA's test book. For $62 and tax/shipping
...which, as you so helpfully pointed out, is the aforementioned $18.50.
I quoted the member price under the assumption that the OP would have at least considered ENA membership at some point, considering that A) it's our major professional organization in emergency nursing, and B) membership gives you a bunch of discounts on educational opportunities, including the CEN exam itself ($220 for members vs. $360 for non-members - there's your $100 membership saved right there, and then some). Apparently that was a bad assumption, for which I apologize.
Back to the discussion at hand. A Google search for "free CEN review" turned up only one site with test questions, and there appeared to be only 17 of them. Everything else I found fell into the "five free questions, pay to see the rest" category. If cost is an issue, you may be smarter to borrow a review book from your hospital's or local university's medical library. If nothing else, they're bound to have a copy of Sheehy's that you can study from in conjunction with your existing review book. You might also ask your department educator (if available) or your hospital's clinical education department if they have any resources you can borrow.