Most of the good non-magic and magic bows in the game appear as random leveled loot on NPCs and in containers (the higher your level, the nicer the loot is). Glass and Daedric bows do the most damage, but you only tend to find them if your level is 20 or higher. Elven bows are decent at mid levels, and at low levels silver is about the best you can expect. Raid bandit lairs like Vilverin, kill the bandit archers and take their non-magic bows (you can enchant them yourself if you've advanced far enough in the Mage's Guild). There's a really nice magic bow called Hatred's Soul that spawns randomly only inside Oblivion gates. I only know two magic bows you can buy:
Bow of Infliction (buy it at the Archer's Paradox in Bravil - mediocre enchantment but good base damage and doesn't need recharged very often).
Bow of Infernal Frost (if you're a member of the Dark Brotherhood, buy it from M'raaj-Dar at the sanctuary, good base damage and good enchantment but it needs recharging pretty frequently so carry lots of soul gems if you use it).
There's also a sweet bow you can get as a quest reward (spoiler):
If you fulfill the contract and also meet the special conditions for the Dark Brotherhood quest The Lonely Wanderer, you will get Shadowhunt as a bonus (good base damage and powerful enchantment).
You could also try searching the equipment database here:
The Elder Scrolls IV: Oblivion @ GameBanshee