Poisons are allowed to be used by the game rules, but they are illegal in character, and as such cannot be freely purchased.

By default, no poisons may be purchased at character creation. However, if your character has the Poison Use ability (through a class feature for example), they may start owning one dose of a poison of their choice (paid for normally from starting gold).

Poison may not be purchased normally after character creation, but it can be gained as loot from adventures, traded with other players or purchased from the Thieves Guild once you have gained their trust.