Instructions
Go to Route 208 and enter the house on the route. Talk to the Berry Master, who's standing next to his wife. Every day, he'll give you one random berry. If you diligently visit the Berry Master every day, he'll give you a Mago berry at least once.
Head to Routes 215, 222 and 221. Interact with the patches with grown plants. On Routes 215 and Route 221, you'll find one Mago berry. You'll find two Mago berries on Route 222. If you want more than four Mago berries, plant these berries instead of using them.
Start Pokemon Sapphire, Ruby or Emerald and attach a Mago berry to any Pokemon. Go to any Pokemon Center and place the Pokemon in the PC. Put Pokemon Emerald into your DS and start Pokemon Diamond, Pearl or Platinum. Go to Pal Park, catch the Pokemon and take the Mago berry off it.
Plant a Mago berry in any empty patch in the Sinnoh region. Use Growth Mulch on the empty patch to make the berries grow more quickly. Water the patch every 2.5 to 5 hours to make the Mago berries grow faster. The more you water, the more berries per harvest you'll get.