At chapter 14 there are secret shops where you can buy them. Which route are you on, Eirika or Ephraim?
On 14 there are secret shops on the red squares. One of the pictures is Eirika's route, one is Ephraim's.
Just have a unit stand on the red square while holding the Member Card item. You get the member card when you recruit Rennac or kill him.
There's also one in chapter 19 which sells 'em.