2013 Altima

My wife drives a 2013 Altima. We are going to be moving and I don't want to install a trailer hitch. I have been looking for a roof top cargo carrier. The way I understand it the box is mounted to rails attached to the roof. The box is easy to find, seems like they all mount pretty much the same way, clamping to rails. My question is which rails to use and where to get them? I checked out Thule website, they didn't list anything for Altima. I checked out Yakima website and found they make something for Nissan but it's very confusing which one I need. "HELP"...... I need a carrier by the end of the month.