I'm not going to tell you what you "should" be buying, because that is a matter of your own level of comfort, and your reasons for going organic. My family chose to go no-plastic with ours, but we do almost everything organic and unbleached, including cloth diapers. If you decide you don't mind the plastic, of course it is OK.
We chose the Naturepedic brand, and it works very well. It is just as waterproof as any plastic, and we love it. It does add to the price considerably, but it's a great product.
http://www.naturepedic.com/products/pads/organicmattressp...