If you have a gameshark advanc - Pokemon Sapphire
If you have a gameshark advanced (not sp) I can help you out here.
First, enter the master (M) code.
This next code will give you an Eon Ticket in your FIRST SLOT of your PC. Make sure the item in the first slot is something junk - like some type of MAIL.
Now you're in for a rude awakening. The Eon Ticket doesn't work! Enter this code to activate it. Once this code is entered you can talk to the lady in Slateport/Lilycove ferry harbors to travel to South Island.
ENABLE EON TICKET
Now this is a bonus because I like you people. This code will allow you to repeatedly use the rock, so if you don't catch Latios the first time you'll get another shot...or you can do what I did and capture many Latios.
Now to help you out with it's capture, you can also use a masterball code. Just note that like the Eon Ticket code, the Masterball codes will replace the items already in the first slots.
MASTERBALL 1ST PC SLOT
MASTERBALL 1ST ITEM PACK SLOT