Does anyone know what this bolt is called or if it’s possible to buy a replacement without buying an entire caliper?

Its a pad adjuster. Unless its a big brand you wont be able to get hold of one. Even if its a known brand it might be sketchy getting one too, as mechanical brakes are so cheap spares really arent that much of a thing.

A bike shop might have some lying around in old calipers, but no guarantee they will fit.