If you can't connect with Martin as described above, call the direct number and ask for Guitar Makers Connection. They will ship you a neck and whatever else you want (fret board, etc).
https://www.martinguitar.com/1833-shop/#!/c91
Last price I saw was last August; dovetail necks for $40 to $65. They are factory "rejects" so may have a pin knot or other small blemish.
