It's called an "FSA Seatpost saddle rail clamp screw", and can be purchased from FSA, but probably only as part of a complete seatpost.
You might be able to hack a repair using a conventional screw with a matching pair of male and female spherical washers. These you can find in brake shoe kits.
Does this type of screw as used in an FSA Seatpost saddle rail clamp have a specific name and does anyone know where spares could be purchased?