The studs were originally welded to the C pillar and I've never seen them sold anywhere. A trick is to use self tapping screws (try to get stainless) as new "studs". The screw head will hold the emblem on, you just need to turn the screw to adjust the height until the emblem fits snug.
Keep in mind the emblems should be installed so the horse runs torwards the front of the car. That's how they were from the factory although you may find some on restored cars running the wrong way.

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1968 Mustang coupe, Acapulco Blue, 289 2v, C-4, Power Steering.
Hopefully a 1969 convertible or Sportsroof (non Mach or Boss) for next Mustang project
Trying to find my father's 1973 Mustang Grande he bought brand new. 3F04F126773 last known registration and title was in New Jersey, 1982.