Where do you find the move thief in Pokemon sapphire?

You can find the Move Thief TM in Pokemon Sapphire in the Battle Frontier. Specifically, you'll find it in the Battle Tower.

Here's how to get it:

1. Complete the main story of Pokemon Sapphire. This includes defeating the Elite Four and becoming the Champion.

2. Travel to the Battle Frontier, located south of Ever Grande City.

3. Enter the Battle Tower.

4. Win at least 5 battles within the Battle Tower.

5. Talk to the Battle Tower staff after winning your 5th battle. They will offer you a reward, and one of the possible rewards is the Move Thief TM.

Note: The specific reward you get is random, so you may need to win more than 5 battles to get Move Thief.