You can't fly in space in Pokemon Emerald.
you have to go to space...
you can get one from a man in moss deep space center in Pokemon emerald.
In Mossdeep City.
There is no space center, that is in ruby, sapphire and emerald.
In Pokemon Ruby/Sapphire/Emerald in Mossdeep City.
In Mossdeep City, to the southeast.
Northwest of the Mossdeep Space Center.