The jungle town of Mideel is located on an island on the southeast side of the map. It can only be reached when you have access to the Highwind.
The story of the game takes you to Mideel very shortly after you obtain the Highwind which is when Tifa, Barret and the rest of the team make their escape from Junon. The team first arrive while they are searching for their leader, Cloud, and they return for a second visit to Mideel when they come to pick up Cloud after they’ve obtained a few pieces of Huge Materia.
Check out the first Mideel walkthrough page and the second Mideel walkthrough page for more information.
There are a number of shops in Mideel where you can purchase items:
The map below shows the location for Mideel on the World Map: