Instructions
Mr. Mime
Travel to Route 24 or Route 25. Walk until you encounter an Abra. Use a move such as "Mean Look" or "Sleep Powder" to prevent Abra from fleeing.
Attack Abra using physical attacks but do not make Abra faint. Throw any kind of Pokeball; however, Great Ball and Ultra Ball will be the most effective. Keep Abra asleep or otherwise weakened to prevent Abra from fleeing battle by using "Teleport." Capture Abra.
Go to Route 3. Enter the house on Route 3. Trade Abra with a nonplayable character inside the house. Save the game after you receive Mr. Mime.
Trade with Link Cable
Plug a link cable into two Game Boy Advances with either Pokemon Fire Red or Leaf Green carts. Turn on Game Boy Advances. Go to a Pokecenter. Enter the Cable Club.
Talk to the receptionist at the far right and select "Trade Center." Wait for both players to be connected. Browse the Pokemon list and select "Mr. Mime." Trade Mr. Mime for another Pokemon.
Complete the trade. Exit the Trade Center. Save the game. Turn off power to both Game Boy Advances. Remove the Game Boy Advance link cable.