Nowhere.
trade
trade him
You can not catch it in leafgreen,but in firered you can.
You can catch Mareep in Altering Cave.Mareep is not available in firered you must trade one from emerald or colosseum.
in 4-7 islands
Since you can't get mareep in firered trade one from emerald.
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't catch Flaafy, but you can catch Mareep at the Valley Windworks.
You'll need to trade for it.
ampharos you mean right, well you can't unless you get a mareep from emerald.
To get Pokemon firered on PC download a vba and Pokemon firered ROM
You can catch a Ho-Oh at Navel Rock in Pokemon FireRed.