Like kyle said, slug your bore. Your 9mm will most likely come out to .355 or .356. GENERALLY a good place to start is purchasing a mold and sizer that will give you a finished bullet around .001" over your groove diameter. Undersized bullets tend to lead barrels. Fit is the most important aspect of having success with lead bullets. For a top punch reference for Lee molds check out Castpics.net and look on the left hand side of the screen for top punch charts. It will tell you which top punch will best fit the Lee molds. DO NOT let anyone pick out your stuff at Midway. BTW, if you have a .358 mold and you decide you need a bullet at .356 for your 9mm, you can indeed size it down .002 with no ill effects. Just make sure that the bullet will function in a semi-auto (maybe a light 38 spl bullet [.358 sized down to .356] in a round nose configuration may work, for example). But, I have found that if you try to sized down the tumble lube style bullets too much you will lose the micro-grooves and the bullet will hold less LLA. Good luck!
~NRA LIFE MEMBER~
~NRA CERTIFIED INSTRUCTOR~