no Mareep can only be found in silver and gold so it must be traded. You can get Mareep in Routes 32, 42 and 43 in the other games so..
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you can't, its only possible to catch him in Pokemon silver and gold on route 6
There aren't any in the wild, you'll have to trade it in from either Gold or Silver.
You'll need to trade for it.
On route 29 i cant remember if it's night or day when you can. Good luck!