You should complete the Hoenn sidequests and when you are done, there a 1.50% chance of getting a red orb.
https://wiki.pokemon-vortex.com/wiki/Sidequests#Hoenn
Here is the list of rewards which you can obtain by completing the hoenn sidequest.
Otherwise you have to buy a red orb through Pokebay auctions (20,000,000 - 25,000,000 mil worth)