Don't try to find high runes. Honestly, I've played this game for nearly 6 years and I have NEVER found a high rune before. Hell Countess has an extremely small chance to drop a high rune, and I believe the highest she can drop is an Ist or Lo rune. I usually run countess for the keys, but occasionally she will drop me an ort or an amn :)
Countess is good, she practically always drops runes. I've gotten an Um and an Ist from her.
I also got an Ist from the chest in the bottom of The Pit - but that was the only rune I've ever seen drop there after a lot of runs, so that was probably a fluke.
I find a lot of runes when I don't try to find them too. Just doing some stuff for fun - killing diablo, heading through the sewers below kurast, etc. You'll be surprised what interesting things drop, including runes.
I also read somewhere that the chests near the bonfires in lower kurast are supposed to have good rune drop rates - but after a lot of runs there (like 50 or so) I've never seen a single rune drop at all - it's probably a similar situation to my lucky drop in The Pit described above.