The secret shop is located in Scabellum. To access it, just get Deneb as a witch in your party, and the secret shop is automatically open. When you enter the shop she will be the one that enters instead of Alphonse (unless you don't have her with you anymore). She will talk to the pumpkin head at the counter, a.k.a. Pumpky, and he will sell many different items that are not
accessable from anywhere else in the game.
The items for sale are: Candy Helm,
Chocolate Shield, Candy Armor,Fur Coat, Sacrificial Doll, Sacred Stone of Bliss, Firecrest*, Orb,
(you can get Deneb and therefore the secret shop by, my
earliest, level 4).
*To get the Fire Crest, you need to sell 5 glass pumpkins in the secret shop. Now, you can get glass pumpkins in several different places.
There are 3 Glass Pumpkins available on the map. In case my descriptions are confusing, I also have the coordinates there to be followed from the bottom of the map, to the right, then to the left.
Lutra on the very top left side, 3 spaces from the very top corner; next to a small ledge. (13,10)
Vespa on the left side of the rock toward the middle right side of the screen.
Gryllus on the right side of the mini-waterfall in the middle. Halfway between the land and the mini-island. (5,7)
There is another that is available through Glycinia. When you get her, she
will come with one equipped.
Even if you get all of those, you still have one left that you need to get. This can be aquired by doing quest mode 2 and completing "defeat all" in 1-12 turns, or "defeat leader" in 1-7 turns. You may have to do this a few times, you are not guaranteed to get it every time.
NOTE: You do not have to sell all of the glass pumpkins at the same time.