You can find poison, but loot (Well, loot you find in crates and barrels and such in dungeons) is usually keyed to your level, so if you're not high level you'll find crappy poisons, I've only found a "Weak poison of Paralyze" or somesuch. 3 seconds of paralyze.
Your best bet is to work up your alchemy skill nice and high by collecting lots of ingrediants. Then you can tell which ingrediants have negative effects and, then you can get alchemical equipment like a mortar and pestle, albemic, calcinator, etc. And then make your own, more effective poisons.