If you are referring to the machine screw that holds the arms onto the spline then any "real" hardware store should have them and they are probably metric. A hobby shop should have them also as some other servo make should fit. If you are looking for the ball screw then it looks like they sell them in pairs AQ1425....not sure who carries them.
http://manuals.hobbico.com/ace/acem8126-manual.pdf