The 1st is found next to the Outfitter the first time you meet them.

There are 4 found in Greed's offices. You will need most of the lucky cats to get to the very end platforming section.

The 2nd and 3rd are found as you progress through the rooms and they're hard to miss.

The last 2 are at the end of the final platforming section.

The 1st can be found on the ship behind the Outfitter when you arrive to the main island.

The 2nd can be found on this island, just off the beach is a small rocky outcrop.

The 1st can be found by going to this teleport and looking down you will see a vertical-moving lift.

Taking the lift down you will find a platforming section with spinning gears. Go across these to find the yellow briefcase.

(You can also get through the back entrance by jumping over the lights onto the top of the door, and then going in.)

The 2nd can be found by going to this teleport. The briefcase is by the group of NPCs.

The 1st briefcase can be found above a branch just in front of the entrance to Club Luxuria.

From that same teleport, looking towards Club Luxuria, there are some ledges to the right. You'll find some lizards that attack you and there's a breakable wall you can smash open. In here you'll find the 2nd yellow briefcase in a small pit with a few silkworms.

The 1st can be found at this teleport. There's a small garden maze and the briefcase is found at the end.

The 2nd can be found by going to this teleport. Go up to the escalators and then turn around. The nearest tower on your right is where the briefcase is at.

There are 3 other costumes to get: 2 are awarded when you beat the final boss and the 3rd when you beat the final boss a second time.