Instructions
Complete the seventh team battle near the Eastward area on the map. Go through the Shoal Cave area and head South until you reach Mossdeep City.
Talk to the first character you run into by pressing the "A" button on your GameBoy Advance. The character will teach you dynamic punch to use for your Pokemon.
Go to the space center and talk to the sailor character. The sailor will give you the sun stone item.
Go into the gym building in Mossdeep City and beat all of the Team Magma trainers in the gym. Go to Steven's house after you beat the final trainer. Talk to Steven and he will give you the dive ball.
Go to the Poke Mart in Mossdeep City and go to the counter. You can purchase another dive ball, called the HM 06 ball, for $1,000.