If your Mana is full you can restock your potions for free by playing MiniGames. I refuse to purchase a potion from that lady in the fair grounds because things in Wizard101 are expensive enough without paying 150 gold per potion in between each dungeon on top of it. All you have to do is complete level two on most of the games in order to earn enough reward to fill a potion bottle. As far as where potions can be purchased in KT, I don't know. I don't think there is a place. Someone correct me if I am wrong.
Sadly, there is not a potion vendor in krokotopia :(, but it would be a great idea to have one in every world! Then again, you can still be able to play those mini games in order to get a potion refill.
I actually am not so sure my self. The first time I entered Krokotopia I thought I saw a street vender that selled potions on a roof of a building. I may have mistaken it for the guy on top of the seceret shop that sells you summon cards. Anyway I do remember buying a fill all potions in Krokotopia but I can not find it. I just stick to the potion make in the fiargrounds at wizard city. Also my guy is Daniel starstone. Currently sneaking into the thunder chamber in Krokotopia. Recently gained acces to the Krokosphinx. If you have a friend that can go to the Krokosphinx or the storm chamber then you can teleport to them. But if they are in a world like Mushoo(Did I spell Mushoo wrong) you can not teleport to them.