Shotgun/Directional Microphone?

I'm looking for a shotgun/directional microphone for my Nikon d5100. I'm going to be using it for videos so I need one that will eliminate any AF noise (auto focus). What should I get?

A couple of things.

One to eliminate that noise the best thing to do is learn manual focus. It's really the only way to shoot video anyhow. I did a career in broadcast television video work and there's no such thing as auto focus.
That's a gimmick at best and normally doesn't work very well besides causing noise in your audio.

I've been hearing pretty good stuff about the Rode brand. I don't own one but the best I've seen was about $150 and they do come in lower priced models.

Here's a good place to start shopping. Http://www.bhphotovideo.com/