Potions, Poisons, Acids, Oils, and other such concoctions can be bought from Apothecaries, Toxicologists, Alchemists, Herbalists, and similar artisans. They can also be crafted through the correct application of the relevant tools and ingredients. 

All concoctions use the Flask/Satchel Usage Die or Vial/Pouch (T, 0.2) for slotted inventory purposes.

It is critical to note that while an ingredient used in a small dosage or treated in a particular way may be used to ease pain and speed up recovery (potion), that same flower may be used to dull the senses, induce psychoactive visions, or bring about an untimely demise (poisons). To avoid any confusions, as a general rule, Restoratives are used to recover, Toxins are used to inflict, and Modifiers are used to prevent or change the state of something. To better distinguish these, you'll find them separated by category below.

Restoratives: Speed up recovery, restore spell slots, remove inflicted conditions, etc.

Toxins: Deal damage, reduce maximum hp, interfere with spell casting, inflict conditions, etc.

Modifiers: Grant abilities, modify weapon damage, amplify damage mitigation, grant condition immunity, etc.