Fashions are available from many sources. Some of the common ones include the cube points shop, exchange events in the ‘Rewards’ window, and the egg smash event. There are also some other events that offer fashions. Sometimes, fashions are given just for recharging ambers, but there are probably a few ways of getting fashions through ambers – I don’t know them, though, and wouldn’t use them anyway. The prime set is available in the cube points shop at the moment (and has been for a while), so it can be obtained by opening the cube enough times. However, cube isn’t usually available to people without spending ambers on it, unless there are cube chances in either the login rewards or in a growth contest. The prime set might also appear in another event, but those usually change each time.
However, fashions sets aren’t necessarily related to your character’s appearance. Your character can wear things from different sets, according to what you prefer or find more meaningful.