Mareep can be found on Route 12 in horde battles. They cannot be found outside of a horde battle.
Source: Found them myself.
The only one you find in the wild is Mareep.
It is found on Route 12
U can get mareep on route 12 in hordes the best wait to get it is using sweetcent or honey .But other than that there no way to get his evolved forums unless you use GTS.
Hope I helped.