Currently, you can only get sapphire units and some other bonuses like free SCs from serial codes I believe. The serial codes are available in Aigis medias such as magazines, the light novels, and the character profile books and Aigis-related events. Sapphire units are between gold and platinum rarity from what I heard.

I'll leave the past serial code question to people who actually used them before. But I did buy one of the light novel volumes recently for her: http://seesaawiki.jp/aigis/d/%c9%b1%...d1%a5%c6%a5%eb