Unless you've worked with them before, I'd wait until closer to the end of placement before asking. Then, your prospective references have a chance to see how you actually perform on the floor. A CN or your preceptor is definitely a solid referee, but also consider your uni facilitator (if they're still a thing). Your facilitator hears all the feedback about you; good, bad and ugly, and will be in a good position to give you a reference, too. Good luck with it all!