How do you use a master seal on fire emblem shadow dragon?

Answer 1

You don't find them anywhere. As the game progresses, you will get them one by one. I believe you get the second one on the level where Pent and Louis join, and the third at the last chapter of the game. I'm not sure when you get the first one though.

Answer 2

I think you have to get a unit to level 10 or above to use it.