Instructions
Talk to the little girl inside the Contest House in Slateport City. She will give you a set of PokeBlocks, which allows you to gain access to the Safari Zone.
Travel to the Safari Zone, near Mt. Pyre on Route 121. Walk inside the Safari Zone building and talk to the man behind the counter to enter the zone. It costs $500 each time you enter the Safari Zone. You will be given 30 Safari Balls.
Walk around the Safari Zone near the entrance and the left side of the Safari Zone to attract a Wobbuffet. You will likely encounter several Pokemon before you encounter a Wobbuffet. Run from any Pokemon that are not Wobbuffet so that you do not waste your limited supply of Safari Balls.
Use a Safari Ball when you encounter Wobbuffet. Safari Zone encounters are different from wild Pokemon encounters as you cannot battle the Pokemon to weaken it, but you can move closer to the Pokemon or use a PokeBlock to distract it. If you get too close the Pokemon may run away or if you use a PokeBlock that the Pokemon does not like it will not be distracted. Continue to use Safari Balls until you capture the Wobbuffet. If it runs away or you run out of Safari Balls, try again until you capture it.