After the events at Hollow Bastion, you'll end up back at Traverse Town. Keep note that all of the basic Heartless have been replaced by tougher ones. The battle level for all of the worlds has increased as well. Upon arrival, Cid will tell you that you need another Gummi piece to take a different route to Hollow Bastion. He also tells you that you should be able to find this Gummi piece in the Secret Waterway.
Head on over to the Alleyway in the Second District. Here, the Gizmo Shop makes a good grinding area. While in the Alleyway, you should be able to go to the Secret Waterway and get the Gummi piece by going over to the mural and talking to Kairi. Talk to her again and she'll give you the Oathkeeper. Use the white trinity behind Kairi to receive an Orichalcum. Go up the stairs and talk to the Fairy Godmother to exchange Fireglow with the Mushu summon.
After you're finished preparing, go back to the First District and talk to Cid. He'll have the piece installed, and you can now move on and go back to Hollow Bastion.