Boethia's Daedric quest-- no question. You have to fight a series of matches against gladiators, and one of them is an archer who's absolutely loaded with enchanted arrows.
This is right up my ally. I will give it a try when I get home from work.
If you're a member of the Dark Brotherhood, and have not advanced the quest line any further beyond, ahem... the cleansing, the Khajiit sells enchanted arrows. He restocks every three days, and these arrows are his mainstays.
Does his stock change every 3 days or does he just refill what he already has?
If you can install mods, here's a small one that adds an archery store next to the stables outside Chorrol:
http://www.oblivionmodwiki.com/index.php/Fletcher%27s_Den
Several types of enchanted arrows there as well as normal leveled ones too. I'm pretty sure the store is open 24/7 as well.
I am on the PS3 so unfortunatly I can't download any mods =(