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
catch a mareep and train it
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 :)
You can not catch it in leafgreen,but in firered you can.
The Kanto or Johto region.
trade
Nowhere.
trade him