How do I get 5 star medals?

5* medals are only obtainable from special events (bonus login items, daily bonus). Apart from that, they are predominantly a pay to play item (you must buy them with money). Because you need 100 of them to promote one officer from 4 to 5 stars, you're better off just purchasing an instant 5 star promotion ticket for 5$ when that event rolls around (which normally occurs once every 2 weeks)