The potion of Sanity is able to be purchased in-game for 2,300 gp after completeing the "Forgotten Places" quest in which you help a priest find the ingrediets to the potion. The potion will provide you with 24-hour sanity protection. (Refer to DM's Powerbook for the Furcht's Insanity power.)
Quote: "Quick! Drink the mysterious black liquid that I just pulled out of my bag... It'll help... Trust me..."