The chance of catching a slakoth with the brave nature is 1/25. I would recommend not obtaining a slakoth with this probability.
To increase your chances, breed a female slakoth with a ditto that has the brave nature. Give ditto the everstone to hold during breeding. The everstone is found in Granite Cave in Dewford Town (source: memory).
Another method is to catch a Pokemon with the ability synchronize AND the brave nature. A good one is abra. Give abra the smoke ball if you have one, and then find a slakoth in the Petalburg Woods.
If you have a hard time, you can always ask around for someone with a spare Gen 3 brave ditto to trade you (I'm sure there's one of those out there).