Normaly you can't, but a use gameshark!!
in the altering cave-six island
mareep can't be obtained in leafgreen but can be caught in emerald to trade it you need two wireless adapters and the national dex.
I don't think you can. You'd have to trade with FireRed or LeafGreen.
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 :)
Stop playing Pokemon a get something better to do
You can't catch Flaafy, but you can catch Mareep at the Valley Windworks.
You cannot catch all Pokemon in Pokemon Leafgreen, you will need help from other versions.