I haven't got the game yet so don't get mad if it's obvious. I really want a Mega Slowbro when I get the game and was wondering if you can catch Slowpoke in Omega Ruby or will I need to transfer it from Y? And if you can catch it could you tell me where? Thanks.
Unfortunately, it can only be found in Mirage Spots (areas 27 and 32) , which can only be found post-game. If you really want a Slowpoke, you could catch one on Route 12 or in Azure Bay in Pokémon X and Y.
For future reference, all Pokémon locations for all games can be found by searching the Pokémon right here on the database, like this: http://pokemondb.net/pokedex/slowpoke
Locations are listed at the very bottom of the page.
EDIT: And here's the location of the Mega Stone for Slowbro
Slowpoke is available at Mirage Cave