After the sixth badge, you have to travel south from Fortree city. This route will have many trainers to battle, and is pretty long. Once you finish it, you will be in Lilycove city. There are a lot of Team Aqua memers there. Talk to all of them and you'll get an idea where to go. Hope I helped! :)
Chat with our AI personalities
Back in the 3rd town where your dad is hope this helps :)
at the gym
Go to the Pokemon league.
Fortree City ( Feather Badge ) Leader's pokemon: Altaria, Swellow, Skarmory, Pelipper
The sixth gym badge (the Marsh badge) is located at Saffron City's gym. It is in Gym Leader Sabrina's possession, so you have to defeat her to obtain it.