you can't
You'll need to trade for it.
Trading is required since it's not obtainable in Pokémon Crystal.
Mareep will evolve at Level 15 in Pokémon Crystal.
you can't, its only possible to catch him in Pokemon silver and gold on route 6
with a pokeball
You can either catch a mareep, breed a mareep, trade a mareep or go to the global trade station and ask 4 a mareep/ Ampharos :)
catch a mareep and train it
You can not catch it in leafgreen,but in firered you can.
In Pokémon Crystal, Ampharos is the evolved form of Flaaffy, which itself evolves from Mareep. You can catch Mareep in the National Park, as well as on Route 34 and Route 42. After obtaining Mareep, you can evolve it into Flaaffy at level 15 and then into Ampharos at level 30. To find Ampharos directly, you will need to encounter it in the wild in the later rematches in the game or through trading, as it is not available in the wild in its final form.
The Kanto or Johto region.
trade
Nowhere.