Ages ago, the only place I found that sold 3 core shielded/screened wire in quantities smaller than complete reels was Farnell, but Polevolt also sell it by the meter (£1.64 + vat + post, part number SCN0.5/3).

I wouldn't use any wire smaller than 1.00mm^2 for the rest of the engine loom (that's the size fitted by Ford originally unless individual circuits required larger) unless you're proposing to use a more "exotic" wire type.