The Poison Shop in SQ has poisons for lvl20 also, the merchant is up stairs in the same building.
you have to decide if you want high damage over time poisons with low first strike or low damage over term poisons with with high first strike
i personaly use either Red and Purple for harder fights or Red and Yellow for fast fights, i forget the names sorry
when you are lvl 30 you get to buy the poisons from the merchant on the first floor of the poison shop, the colors are the same just diffrent names of the moves