For unlimited potions, go to Timber on the bridge and you will see a man (you must have Quistis in your party). He will give you a potion and run away. If you leave the screen and re-enter it, he will be there again and give you another potion and run away. Keep repeating this and he will keep giving you potions. (NOTE: He isn't always standing there when you enter the screen. If he isn't, leave and come back until he appears there.)