Thomas's Bazaar is found in Tazmily Village, west of the Yado Inn. Thomas runs the shop. You can pick up random things there from a Nut to the rare Better Stick. You can also get equips, bombs, and other stuff. Also, the only time you can actually get something from the bazaar is in Chapter 1, the second half of Chapter 2, and certain portions of Chapter 3. In the first part of Chapter 2, it's nightime, and in Chapter 3, Thomas won't let Salsa touch the merchandise unless Fassad is in the party.
Thomas now works at the Clayman Factory, so his son Nichol is there. He sells:
Like everyone else, Nichol leaves for the big city in chapter 7, so you can't get items there anymore.