The pok?block case is an item in "Pok?mon Sapphire" that can be used to create food for the Pok?mon that increases your Pok?mon's performance in Pok?mon contests. To obtain the case, you must go to Slateport City and speak with a specific character.
Head to Slateport City
Access Slateport City after Mr. Briney's Wingull has been saved from the Team Aqua Grunt in Rusturf Tunnel. Mr. Briney then gives you a ride in his boat to either Dewford City or Slateport City.
Go to the contest hall
In "Pok?mon Sapphire" and "Pok?mon Ruby," there is a contest hall at the top of the city. Go there and enter the hall. In "Pok?mon Emerald," this building was replaced by the battle tent.
Speak to the small girl
There are many characters who are wandering around the contest hall in Slateport City. One of them should be a small girl. If you talk to her, she gives you a pok?block case. The contest hall in Slateport City is a hyper rank contest hall, which means that even if you do have the pok?block case, your Pok?mon are unable to take part in the contests until you have completed the super rank contest.