From their entry in Monster Core:
Hell, according to some theologians, is a living entity in and of itself. Diabolic dragons, these scholars argue, are just extensions of the plane, living creatures that break off from Hell to enact its will. Whether this is true or whether diabolical dragons are simply the reborn souls of dragons sent to Hell, the fact remains that these dragons are powerful, cunning, and tyrannical. Every diabolic dragon's goal is to further Hell's will, though how this happens can vary. Regardless of their goals, these dragons always approach newcomers with an unsettling calmness.
Another entry from Monster Core talking about their lairs:
A diabolical dragon's lair is a surprisingly sparse place. Most of these dragons find caverns or large rooms within Hell to claim as their lair but keep relatively little within. A diabolical dragon's hoard is generally made up of soul gems—magical gems designed to contain mortal souls— and infernal contracts. The contracts are ones still in effect that will resolve once the mortal signer's soul reaches Hell and is forcibly placed in a soul gem. While diabolical dragons pass the souls contained in these gems off to other parts of Hell to be used for countless purposes, most dragons hold on to the souls of mortals they find particularly intriguing and call upon the souls to be servants or for other wicked purposes at a later time.
This seems to imply that diabolic dragons are predominantly in Hell. How common are they outside of it, running about in the Universe?