Where can you catch a castform in Pokemon sapphire?

Answer 1

You can't catch a Castform in the wild. So if you want one here's how it goes. On the way to Fortree City, you'll see a big building, this is the Weather Institute. Go inside and you'll see that Team Aqua have invaded, fight your way through the Grunts on the bottom floor and then make your way up the stairs and have a double battle with two more Grunts. Make your way to the west of the room and battle Team Aqua Commander (second in command) Shelley. She'll have some pretty strong Pokemon so if yours are exhausted from the double battle, go down the stairs and find a bed. Battle her and then a Team Aqua Grunt will run in and say they just saw Team Magma pass the Weather Institute and they bugger off. Then the professor will thank you for saving them and the Weather Institute and will give you a Castform.