Is there any particular group of creatures that drops runes more than others?

Not normally, although certain types CAN drop more than others, remember that drops are completely random though, normally the chance builds up over time I suspect.

I have noticed some differences in Rune Drops, For example Runes drop good at Hobgoblins and in Temple, while they dont drop at all in Desert (or very rarely).
Basically, the more you kill, the higher chances for Runes.
More kills -> More chances -> More Runes, so find an area in which you can kill mobs fast.