Secret Shops (秘密の店) appear in most games in the Fire Emblem series, they are hidden in a few chapters which sell rare weapons and items such as promotional items, or stat boosters. However, these shops are hard to find and are well hidden. You need a Member Card to enter them and you must stand on the square they are hidden on. These shops do not appear on the map and they look like ordinary squares unless you step on them with a character holding the member card. Sometimes only certain characters (like Marth) can enter them.
These shops sell a limited amount of a certain item. To get to the shop, you normally have to recruit a rogue/thief with the card (e.g., Rennac). Then find a spot that looks funny, maybe a corner with an enemy next to it or a single spot blocked from view.
The shopkeeper is Anna.