Celtic Heroes

I know I can buy them with plat, but I heard there is a quest where I can get one. Where is this quest?

Dustwither Catacombs. also when you come of level Stonevale. You have to complete the beginning quests, however.

Which quest is it in catacombs? I could a sworn I cleared that area

In Death's Caress at Catacombs theres a quest giver (requires u to be lvl 25 I believe) that asks u to kill :

Disclipe Ducarios
Disclipe Kiara
Avatar of Donn

After ur done u get book of alt and some extra prizes.

Aaaahhhh yes.. I do have that quest. But I am lvl 36, am I strong enough to do it?

You should be, depending on your build, for kiara at least. I would suggest getting a group of friends together. 2 or 3 of you should be able to kill even donn.