i only have 7 and that's on a fighter character. i'm making a rogue/mage so i'll need good arrows.

Stores should stock up once you reach a certain level, as well as bandits, I honestly forgot what level the elven stuff starts showing up though. :/
I have gotten the majority of bandits when I got to that level....er I think it was 7? Don't quote me on that though.

Just found out, Archers Paradox in Bravil should be your best bet.

Another edit, Stores should start selling at level 10, lvl 9 for the random loot.

You'll find more as you level up more. Like killerpenguin said, bandits are a good source.

I didn't see my first ones until level 11. Got them off an archer vampire