Slowbro can't evolve. Only Slowpoke evolves into Slowking.
It can be found on Route 13. You also have a 1% chance of getting one after battle with a Pokemon who has the ability Pickup at level 11-30. Poliwhirl, Politoed, Poliwrath, Hariyama, Slowking, and Slowbro all have a 5% chance of holding a King's Rock.