For a direct replacement you need a hub with the same spoke-hole-circle diameter and the same centre-to-flange distance for both sides. Since it is a road hub, it is likely symmetrical any other symmetrical (non-disk) hub with the same flange width and hole circle should work.
Can I ask why? What are you hoping to gain by doing this? Is the Tiagra hub damaged somehow? I can think of few ways to change your bike that will make less of a difference than removing a new Tiagra hub.