If you mean something that replaces the camera's own microphone, there are none available yet. Generic ones won't work, as NEX-6 doesn't have a standard audio input. Sony probably has a compatible mic in the pipeline but nothing concrete that I have heard of.
I personally don't mind - I'll be using the same setup I've used with Canon 550D: a Zoom H1 digi recorder attached to camera with a cold shoe adapter, recording the audio separately (to be synced afterwards). That way I don't have to deal with any AGC hassles either.