Brake mounting on many bike frames is by recessed sleeve nuts now, those are made in a variety of lengths.

Tektro may be the likely place to look to external nut fixed brake calipers . they are a high volume brand .

Given, single pivot brake the manufacturer makes the center bolt, so you need to get theirs from them.

not much generic ... Asia makes whole brakes for cheap..
economics to just make bolts that cost more than the whole brakes , yea right !

some times recessed fronts become rears . wing it.