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.