I think all scout classes can use poison (I know Rangers and Assassins can). In any case, if you are in Qeynos, then you can buy it in South Qeynos. There is a shop labeled "Poison" Its near the gate that leads to the Tower. Just look for the Alchemist shop where they keep saying "There will be no refunds for caster error in this shop" or "Have you seen that highly explosive bottle I set down? I've seem to misplaced it"... Or something like that.
Anyway, I hope that helps... if you are in Freeport then I really can't help.