At the load screen, press Up, Down, Up, Down, Left, Right, Select, I.
On the data load screen after selecting to load on the main menu, highlight a used out-of-battle save and enter in the following:
right, down, up, left, VI, V, IV, Select, III, I
Press Start after selecting your scenario.
On the intermission setup screen, place the cursor on the shop option in the item menu and enter in the following to enter a secret shop.
Enter the intermission setup screen. Highlight the "Shop" option on the item menu, then press Left, Up, Down, Right, Left, Select, Right, Up, Left, Down, Right, Select, I. You will enter a secret shop with all items.