Did you look it up here at Alla's, using the links on the left?
I clicked over there and found this
Celestial Healing in the level 44 section of cleric spells.
It says there that is is dropped in many places (all Velious zones) by a lot of mobs, and even has "Found as Loot" at the top of the list. There are also some posts below it, that refer to which camps might be the best to get it from.
Good luck getting it, since I guess no one has it for sale on your server. (And if they did, it'd probably be expensive.) You'll definitely be wanting to take a group out to help you get it if you're around level 44.