I've had pretty good luck at a local automotive store (Neill - VIP up here in Portland) and matching the silver paint from my old Motobecane with a touch-up paint bottle with applicator. I haven't seen silver nail polish yet, but I bet it's out there. My only problem with the auto paint is that it's so thin, I need a gray primer beneath it on the bare spots, which I just use model paint after a real light sanding. Close, but not perfect.
Dave Whitney
Portland, Maine