Is there a Pokemon Ruby Destiny Rescue Rangers walkthrough?

Yes, you can! You need all 8 badges. Then, go to Abra in the very top-right corner of Pokemon Square. He will teleport you to South Isle. Follow the path (the only way you can go) and when you get to the point when you can go 2 ways, go up, take a right turn, and you will see Kecleon who sells things called "notes" which are just basically Pokeballs.