Just get her to level 10 and then promote her using a master seal? Pretty sure you can straight away gain "axe" usage for War Cleric and "tome" usage for the other class...she's still pretty weak after her first promotion though, so you got to pair her up with someone else hoping for dual strike assists for Lissa or pick on enemies with low hp.
Around Chapter 12 or 13 -I forget which- you'll be able to buy them, but if I recall correctly you will also find some before that point. Some of the secret shops early in the game will also sell them, but you can't rely on that, because they appear randomly and even when they do show up, they'll only allow you to buy one Master Seal at most. --- "Playing a game for its graphics is like watching pornography for the story." - Kadiroth