They're not fog lights... they're actually the turn signal lights ('71 and '72)... And you should be able to get them from just about any vendor dealing in Mustang restoration parts.
The sport lights in the grill on Mach 1s (and other models with the same grill) are kind of more like parking lights than fog lights as well... they come on with the headlights, but don't really light anything up or blink when the turn signals are used.
Kind of a goofy design. I wish they weren't so integral to how the car looks, I probably would've put the turn signals in the sport light sockets, and use the signal lights under the bumper instead for some serious driving/fog lights (projectors).
But - I'll be doing the stock thing and maybe just stuff some projectors way up under the valance so they're hidden from view but still throw some monster light. Gotta see those critters at night, after all.