in the secret johto area.
You can find Mareep in the extension of the safari zone or in the altering cave. ((To find in altering cave you must use your mystery gift) And to get the extension in the SafariZone you must beat the elite four and champion.
After you beat the champion, the safari zone is then upgraded automatically. Go to the safari zone, there you'll see a path at rightside. Follow the path and you'll find mareep and many other Johto pokemon.
Beat the elite 4, then go to the safari zone. check the area that 2 guys were blocking off. If you have the nat. dex, then you can find shuckle, milktank, mareep...
You have to go to the Safari Area and go to the section which is only accessible through a Mach Bike. It is the most northwestern part of the Safari Zone.
go to the safari zone in emerald
Nowhere But The Safari Zone!
the grass in the rocky section in the newly opened part of the park
after u beet the elite 4, go to the saffari zone a whole new part of it opens. u should see mareep eventully. it took me a while be cus they all fled
Safari Zone.
You can find Pineco in the updated safari zone
In Pokémon HeartGold, Flaaffy can be found at Routes 42 and 43. Additionally you can find and catch Mareep in Routes 33, 42 and 43 and the Safari Zone and then evolve Mareep at Level 15 which will cause Mareep to evolve into Flaaffy.
in ruby, sapphire and emerald u will find the safari zone right before lilycove city