Judging by the fact that you used "accoustical" in place of "acoustic", I'm assuming your knowledge of guitars is minimal. And with a Martin, I wouldn't want to take a chance. Any music store will sell you the peg you need, but I don't reccomend putting it on yourself. With acoustics you want to be very careful with the placement of things like this, as to not compromise the sound quality and resonation.
Brett, April 2006