Are you refering to the knobs on the end of the tuning pegs? Or is it a accustic guitar, and your looking for the pegs that hold the ball end of the string in the bridge?
The tuning peg knobs come in all diferent shapes and sizes. Some are pressed on, some screwed on, some cast right on. they are not normaly replaceable, unless you happan to find a old tuner that uses the same size/style knob. Unless the guitar is a real collector piece, your normaly better off just replacing all the tuners. they are not realy that pricy, and replacing a non screw on one may not be possable.
if its the peg that hold the string in, in the bridge, you can get them at any music store. or if you just need one or two, just about any guitar player that plays accustic probably has a box full of them somewere.