Where can I get an extra picatanny rail for the new ACH helmets they are fielding?

They usually don't have "spares". There is a BIOP that specifies the distribution and how many get issued. During a fielding, the parts are for Force mod. There are a few spares per issue for defects, etc but none in the system for Sustainment that you can order with an NSN. Only they I'd say, is ask the fielding people if there I see a spare you can snag.