Where do I get the teleport spell in Candybox?

There are several different teleport methods, none of which are directly acquirable from the main screen.

1: Teleport Scroll: This is most likely what you mean, teleports the character back to the start of the current screen in-quest. It is one of the random scrolls you can get from the candy merchant for 400 candies.

2: Escape Potion: This potion teleports you out of a quest to the main menu, you do not receive any rewards. These can be gotten from the Wishing Well.

3: G.M.O.O.H. Potion: This potion teleports you to one of four locations, the Forest, the Desert, the Ocean, or the Cow Level. You lose all rewards you have earned, but can earn rewards from the level you've been teleported to. Can be brewed in the cauldron.

4: Enemy Teleport: Functions like a teleport scroll, but is a special feature on certain levels. The gates of Hell, and the mage at the castle entrance cause this.

Scrolls can be purchased from the candy merchant for 400 candies after you purchase the Swords.

The scroll you receive is randomly selected. Before you find the Magician's Hat, you will only be able to get the Fire, Acid Rain, or Teleport scrolls. After you find the Magic Hat, you have an even chance to get all five scrolls.

When you buy a scroll, it has a chance of being a teleport scroll. Keep buying scrolls until you get one. Scrolls become available once you have bought the final diamond sword.