Get Alakazam in Pokémon Emerald

Have your eyes on that powerful psychic Pokémon? Alakazam can make a powerful addition to any trainer's Pokémon party.

Steps

Stock up on Ultra Balls. These are the best kind of Pokéballs. They have the highest rate of catching Pokémon (out of the standard three Pokéballs).
Go to Granite Cave. This is the place where you met Steven.
Search until an Abra pops up. A wild Abra's only move is Teleport, so you have to catch it on your first try or it will Teleport out of the battle.
After you caught it, level your Abra up to level 16. It will then evolve into Kadabra.
Trade your Kadabra onto a different game cartridge. This can be your friend's, another one of yours, or someone else's (that you trust). After Kadabra is traded, it will evolve into Alakazam. Trade it back to your game cartridge.

This will be easier and quicker if you have more than one Game Boy (or DS) and more than one Third Generation game.